Gly 101

Great question. I'm assuming you are referring to minerals which form igneous rocks. The vast majority of the common igneous rock-forming minerals belong to the silicate group, which means that they are based on silicon and oxygen for their basic elemental components. The silicates can be further divided into mafic and felsic - 2 very broad categories that relate to the other elements which occur along with the ever-present SiO2. In any event, from mafic to felsic, the common igneous rock forming minerals would include the following:
Olivine: olive green to black, translucent, with a conchoidal fracture. Olivine phenocrysts are relatively common in some basaltic rocks (like those found in Hawaii), and make an extremely pretty contrast with the black groundmass of the basalt. A semi-precious variety occurs (peridot), which can be cut and faceted like any other gemstone.
Pyroxene: green to black, nearly opaque, 2 cleavages at approx. 90°. Enstatite is a common member of the pyroxene family, and can be found in gabbro and mafic diorites. Pyroxenite, an igneous rock composed totally of pyroxene minerals, is related to ultramafic terrains and is therefore relatively rare at the surface of the earth's crust.
Amphibole: mostly black, forms long, slender crystals with 2 cleavages at 60° and 120°. The most common member of the amphibole family is hornblende, which is easy to identify in diorite, granodiorite, and some granites. Amphibolite is a metamorphic equivalent of basalt, and can contain extremely coarse grained specimens of hornblende.
Feldspar: all have 2 cleavages at approx. 90° and a hardness of 6. Approximately 60% of the earth's crust is composed of feldspar, and I tell my Geology 101 students that it's probably a pretty good idea to be able to identify the various members of the family. The mafic variety (plagioclase) may have striations (very fine "razor-cut" grooves on selected cleavage faces), but not always. The felsic variety (orthoclase) can often be pink and has no striations. Both can be white, which can make a specific determination of which feldspar a bit awkward (especially if there are no visible striations).
Mica: translucent to black (felsic to mafic), with one (1) perfect cleavage, causing it to easily break into thin sheets. The mafic mica is called biotite, with the more felsic member of the family affectionately referred to as muscovite.
Quartz: hard, durable, relatively inert, and no cleavage (but a great conchoidal fracture). Quartz is the last mineral to form in a felsic (granitic) rock, and can generally be found filling in between all of the other minerals. When allowed to cool and crystallize in open space, quartz commonly forms 6-sided (hexagonal) crystals which are highly prized and sought after by many people for a variety of natural (and super-natural) uses.
Click here for additional information on Bowen's Reaction Series, which describes the progression of minerals which are formed as magma is cooled and undergoes a phase change from liquid to solid.

These tables will help you identify almost any rock you're likely to find. Read How to Look at a Rock for help with your observations.
First, decide whether your rock is igneous, sedimentary or metamorphic.
• Igneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava (about igneous rocks)
• Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks (about sedimentary rocks)
• Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica (about metamorphic rocks)
Next, check the rock's grain size and hardness. Then start in the left column of the appropriate table below and work your way across. Follow the links to pictures and more information. If you don't find a match, try another of the three big types.
Grain Size: "Coarse" grains are visible to the naked eye (greater than about 0.1 millimeter), and the minerals can usually be identified using a magnifier; "fine" grains are smaller and usually cannot be identified with a magnifier. (using a magnifier, identifying minerals)
Hardness: Hardness (as measured with the Mohs scale) actually refers to minerals rather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, "hard" rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6-7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3-5.5); "very soft" rock does not scratch fingernails (Mohs 1-2). Igneous rocks are always hard.
Identification of Igneous Rocks
Grain Size Usual Color Other Composition Rock Type
fine dark glassy appearance lava glass Obsidian

fine light many small bubbles lava froth from sticky lava Pumice

fine dark many large bubbles lava froth from fluid lava Scoria

fine or mixed light contains quartz
high-silica lava Felsite

fine or mixed medium between felsite and basalt medium-silica lava Andesite

fine or mixed dark has no quartz low-silica lava Basalt

mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine
Porphyry

coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite

coarse light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite

coarse medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite

coarse medium to dark no quartz; may have olivine
high-calcium plagioclase and dark minerals Gabbro

coarse dark dense; always has olivine
olivine with amphibole and/or pyroxene Peridotite

coarse dark dense mostly pyroxene with olivine and amphibole Pyroxenite

coarse green dense at least 90% olivine Dunite

very coarse any color usually in small intrusive bodies typically granitic Pegmatite

Identification of Sedimentary Rocks
Hardness Grain Size Composition Other Rock Type
hard coarse clean quartz
white to brown Sandstone

hard coarse quartz and feldspar
usually very coarse Arkose

hard or soft mixed mixed sediment with rock grains and clay gray or dark and "dirty" Wacke/
Graywacke

hard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate

hard or
soft mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia

hard fine very fine sand; no clay feels gritty on teeth Siltstone

hard fine chalcedony
no fizzing with acid Chert

soft fine clay minerals splits in layers Shale

soft fine carbon black; burns with tarry smoke Coal

soft fine calcite
fizzes with acid
Limestone

soft coarse or fine dolomite
no fizzing with acid unless powdered
Dolomite rock

soft coarse fossil shells mostly pieces Coquina

very soft coarse halite
salt taste Rock Salt

very soft coarse gypsum
white, tan or pink Rock Gypsum

Identification of Metamorphic Rocks
Foliation Grain Size Hardness Usual Color Other Rock Type
foliated fine soft dark "tink" when struck Slate

foliated fine soft dark shiny; crinkly foliation Phyllite

foliated coarse hard mixed dark and light wrinkled foliation; often has large crystals Schist

foliated coarse hard mixed banded Gneiss

foliated coarse hard mixed distorted "melted" layers Migmatite

foliated coarse hard dark mostly hornblende Amphibolite

nonfoliated fine soft greenish shiny, mottled surface Serpentinite

nonfoliated fine or coarse hard dark dull and opaque colors, found near intrusions Hornfels

nonfoliated coarse hard red and green dense; garnet and pyroxene Eclogite

nonfoliated coarse soft light calcite or dolomite by the acid test
Marble

nonfoliated coarse hard light quartz (no fizzing with acid) Quartzite


Mineral Usual Color Crystals Cleavages Hardness Diagnostic
Biotite
Black Rare 1 perfect 2–3 Cleavage
Calcite
White Common 3 good 3 Acid fizz
Dolomite
White Common 3 good 4 Acid no fizz
Feldspar
White or pink Common 2 good 6–6.5 Hardness
Hornblende
Black Common 2 (60/120°) 5–6 Cleavage
Muscovite
White Rare 1 perfect 2–3 Cleavage
Olivine
Green Common 1 fair 6.5–7 Color
Pyroxene
Dark Rare 2 (87/93°) 5–6.5 Cleavage
Quartz
White Common None 7 Fracture
People don't usually look at rocks closely. So when they find a stone that intrigues them, they don't know what to do, except to ask someone like me for a quick answer. After many years of doing so, I hope to help teach you some of the things that geologists and rockhounds do. This is what you need to know before you can identify rocks and give each one its proper name.
Where Are You?
The first thing I ask a questioner is, "Where are you?" That always narrows things down. Even if you aren't familiar with your state geologic map, you already know more about your region than you suspect. There are simple clues all around. Does your area contain coal mines? Volcanoes? Granite quarries? Fossil beds? Caverns? Does it have place names like Granite Falls or Garnet Hill? Those things don't absolutely determine what rocks you might find nearby, but they are strong hints.
This step is something you can always keep in mind, whether you're looking at street signs, stories in the newspaper or the features in a nearby park. And a look at your state's geologic map is intriguing no matter how little or how much you know.
Make sure you have real rocks that belong where you found them. Pieces of brick, concrete, slag and metal are commonly misidentified as natural stones. Landscaping rocks, road metal and fill material may come from far away. Many old seaport cities contain stones brought as ballast in foreign ships. Make sure your rocks are associated with a real outcrop of bedrock.
There is an exception: many northern localities have lots of strange rocks brought south with the Ice Age glaciers. Many of the state geologic maps show surface features related to the ice ages.
Now you will start to make observations.
Find a Fresh Surface
Rocks get dirty and decay: wind and water make every kind of rock slowly break down, the process called weathering. You want to observe both fresh and weathered surfaces, but the fresh surface is most important. Find fresh rocks in beaches, roadcuts, quarries and streambeds. Otherwise, break open a stone. (Don't do this in a public park.) Now take out your magnifier.
Find good light and examine the rock's fresh color. Overall, is it dark or light? What colors are the different minerals in it, if those are visible? What proportions are the different ingredients? Wet the rock and look again.
The way the rock weathers may be useful information—does it crumble? Does it bleach or darken, stain or change color? Does it dissolve?
Observe the Rock's Texture
Observe the rock's texture, close up. What kind of particles is it made of, and how do they fit together? What's between the particles? This is usually where you may first decide if your rock is igneous, sedimentary or metamorphic. The choice may not be clear. Observations you make after this should help confirm or contradict your choice.
Igneous rocks cooled from a fluid state and their grains fit tightly. Igneous textures usually look like something you might bake in the oven.
Sedimentary rocks consist of sand, gravel or mud turned to stone. Generally they look like the sand and mud they once were.
Metamorphic rocks are rocks of the first two types that were changed by heating and stretching. They tend to be colored and striped.
Observe the Rock's Structure
Observe the rock's structure, at arm's length. Does it have layers, and what size and shape are they? Do the layers have ripples or waves or folds? Is the rock bubbly? Is it lumpy? Is it cracked, and are the cracks healed? Is it neatly organized, or is it jumbled? Does it split easily? Does it look like one kind of material has invaded another?
Some kinds of structural features, like concretions, folds, ripples and slickensides, appear in this gallery of geologic features and processes.
Try Some Hardness Tests
The last important observations you need require a piece of good steel (like a screwdriver or pocket knife) and a coin. See if the steel scratches the rock, then see if the rock scratches the steel. Do the same using the coin. If the rock is softer than both, try to scratch it with your fingernail. This is a quick and simple version of the 10-point Mohs scale of mineral hardness: steel is usually hardness 5-1/2, coins are hardness 3, and fingernails are hardness 2.
Be careful: a soft, crumbly rock made of hard minerals may be confusing. If you can, test the hardness of the different minerals in the rock.
Now you have enough observations to make good use of the quick rock identification tables. Be ready to repeat an earlier step.
Observe the Outcrop
Try to find a larger outcrop, a place where clean, intact bedrock is exposed. Is it the same rock as the one in your hand? Are the loose rocks on the ground the same as what's in the outcrop?
Does the outcrop have more than one kind of rock? What is it like where the different rock types meet each other? Examine those contacts closely. How does this outcrop compare to other outcrops in the area?
The answers to these questions may not help in deciding on the right name for the rock, but they point to what the rock means. That's where rock identification ends and geology begins.
Getting Better
The best way to take things further is to start learning the most common minerals in your area. Learning quartz, for instance, takes only a minute once you have a sample.
A good 10X magnifier is worth buying for close inspection of rocks. It's worth buying just to have around the house. Next, buy a rock hammer for efficient breaking of rocks. Get some safety goggles at the same time, although ordinary glasses also offer protection from flying splinters.
Once you've gone that far, go ahead and buy a book on identifying rocks and minerals, one you can carry around. Visit your nearest rock shop and buy a streak plate—they're very cheap and can help you identify certain minerals.
At that point, call yourself a rockhound. It feels good

At the most general level, rocks fall into three great categories, and they're pretty simple to tell apart. You won't even need a rock hammer or hand lens, though those are fun to have.
Igneous rocks are the first great class.
Origin of Igneous Rocks
Igneous rocks begin as hot, fluid material, and the word "igneous" comes from the Latin for fire. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.
Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting. (To learn more about how igneous rocks form, see About Volcanism.)
People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field, and this article is only the barest outline.
Igneous Rock Textures
Tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains, or an aphanitic texture. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Both intrusive and plutonic rocks have phaneritic texture.
Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of a piece of bread as a similar example.
In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called phenocrysts, and a rock with phenocrysts is called a porphyry; that is, it has a porphyritic texture. Phenocrysts are minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history.
Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture. And pillow lava is a lumpy formation created by extruding lava underwater.
Igneous Rock Types: Basalt, Granite and More
Igneous rocks are classified by the minerals they contain. The main minerals in igneous rocks are hard, primary ones: feldspar, quartz, amphiboles and pyroxenes (together called "dark minerals" by geologists), and olivine along with the softer mineral mica.
The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.
These two categories cover the great majority of igneous rocks. Ordinary people, even ordinary geologists, use the names freely. (Stone dealers call any plutonic rock at all "granite.") But igneous petrologists use many more names. They generally talk about basaltic and granitic or granitoid rocks among themselves and out in the field, because it takes lab work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow subsets of these categories. (Get deeper into classification)
But a few of the less common igneous rock types can be recognized by non-specialists. For instance a dark-colored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. And there is a suite of ultramafic rocks with even more dark minerals and even less silica than basalt. Peridotite is the foremost of those.
Where Igneous Rocks Are Found
The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic. (more on arc volcanism)
The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually. In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle. The behavior and histories of granitic rock bodies are among geology's deepest and most intricate mysteries.
Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types.
Clastic Sedimentary Rocks
The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud and sand and gravel and clay. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocks. (Feldspar and other minerals may also be in sediment if they have not had time to break down.) These are carried away by water or wind and laid down in a different place. Sediment may also include pieces of stones and shells and other objects, not just grains of pure minerals. Geologists use the word clasts to denote particles of all these kinds, and rocks made of clasts are called clastic rocks.
Look around you at where the world's clastic sediment goes: sand and mud is carried down rivers to the sea, mostly. Sand is made of quartz, and mud is made of clay minerals. As these sediments are steadily buried over geologic time, they get packed together under pressure and low heat, not much more than 100°C. In these conditions the sediment is cemented into rock: sand becomes sandstone and clay becomes shale. If gravel or pebbles are part of the sediment, the rock that forms is conglomerate. If the rock is broken and recemented together it is called breccia. See examples of all these in the Sedimentary Rock Gallery.
It's worth noting that some rocks commonly lumped in the igneous category are actually sedimentary. Tuff is consolidated ash that has fallen from the air in volcanic eruptions, making it just as sedimentary as a marine claystone. There is some movement in the profession to recognize this truth, although I still observe convention by mentioning tuff in About Igneous Rocks.
Organic Sedimentary Rocks
Another type of sediment actually forms in the sea as microscopic organisms — plankton — build shells out of dissolved calcium carbonate or silica. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). These are called organic sedimentary rocks, although they're not made of organic material as a chemist would define it.
Another type of sediment forms where dead plant material builds up into thick layers. With a small degree of compaction, this becomes peat; after much longer and deeper burial, it becomes coal. Coal and peat are organic in both the geological and the chemical sense.
Although peat is forming in parts of the world today, the great beds of coal we mine formed during past ages in enormous swamps. There are no coal swamps around today, because conditions do not favor them. The sea needs to be much higher. Most of the time, geologically speaking, the sea is hundreds of meters higher than today and most of the continents are shallow seas. That's why we have sandstone, limestone, shale and coal over most of the central United States and elsewhere around the world's continents. (Sedimentary rocks also become exposed when the land rises. This is common around the edges of the Earth's lithospheric plates, and for more about that, see Plate Tectonics in a Nutshell.)
Chemical Sedimentary Rocks
These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan.
In some cases chert can also form by precipitation. This usually happens below the sediment surface, where different fluids can circulate and interact chemically.
Diagenesis: Underground Changes
All kinds of sedimentary rocks are subject to further changes during their stay underground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals. These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphism (although there is no well-defined boundary between the two).
The most important types of diagenesis involve the formation of dolomite mineralization in limestones, the formation of petroleum and of higher grades of coal and the formation of many types of ore bodies. The industrially important zeolite minerals also form by diagenetic processes.
Sedimentary Rocks Are Stories
You can see that each type of sedimentary rock has a story behind it. The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils, marks left by water currents, mudcracks or more subtle features seen under the microscope or in the lab.
From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.
Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks you can recognize, in very direct ways, what the world was like in the geologic past.
Get more help identifying these and other sedimentary rocks in the Rock Identification Tables and the Sedimentary Rock Gallery.
Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape-changed")
. Metamorphism acts at two scales, the regional scale and the local scale.
The Four Agents of Regional Metamorphism
Heat and pressure usually work together, because both rise as you go deeper in the Earth. At high temperatures and pressures, most rocks break down and change into a different assemblage of minerals that are stable in the new conditions. The clay minerals of sedimentary rocks are a good example. Clays are surface minerals, which form as feldspar and mica break down in the conditions at the Earth's surface. With heat and pressure they slowly return to mica and feldspar. Even with their new mineral assemblages, metamorphic rocks may have the same overall chemistry they had before metamorphism.
Fluids are an important agent of metamorphism. Every rock contains some water, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated by clay minerals as they change back to feldspar and mica. This water can become so charged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica (forming chalcedony) or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. That process, which changes a rock's chemistry rather than just its mineral assemblage, is called metasomatism.
Strain refers to any change in the shape of rocks due to the force of stress. Movement on a fault zone is one example. In shallow rocks, shear forces simply grind and crush the mineral grains (cataclasis) to yield cataclasite. Continued cataclasis yields the hard and streaky rock mylonite.
Under greater heat and pressure, when metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called foliation, is important to observe when identifying a metamorphic rock. As strain increases, the foliation becomes more intense, and the minerals sort themselves into thicker layers. The foliated rock types that form under these conditions are called schist or gneiss, depending on their texture. Schist is finely foliated whereas gneiss is organized in wide bands of minerals.
The Basic Metamorphic Rock Types
The sedimentary rock shale metamorphoses first into slate, then into phyllite, then a mica-rich schist. The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented. Thus the sedimentary rock sandstone turns to quartzite. Intermediate rocks that mix sand and clay — mudstones — metamorphose into schists or gneisses. The sedimentary rock limestone recrystallizes and becomes marble.
Igneous rocks give rise to a different set of minerals and metamorphic rock types; these include serpentinite, blueschist, soapstone and other rarer species such as eclogite.
Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy, and the result is called migmatite. With further metamorphism, rocks can be turned into something hard to tell from plutonic granites. These kinds of rocks give joy to experts because of what they say about deep-seated conditions during things like plate collisions. The rest of us can only admire the laboratory skills needed to make sense of such rocks.
Contact or Local Metamorphism
A type of metamorphism that is important in specific localities is contact metamorphism. This usually occurs near igneous intrusions, where hot magma forces itself into sedimentary strata. The rocks next to the invading magma are baked into hornfels or its coarse-grained cousin granofels, another subject for specialists. Magma can rip chunks of country rock off the channel wall and turn them into exotic minerals, too.
Surface lava flows and underground coal fires can also cause mild contact metamorphism of the same degree as occurs when baking bricks.

How to Identify Minerals: 10 Steps to Mineral Identification
Step 1: Pick Your Mineral
Learning mineral identification is like learning to cook. You begin by following step-by-step procedures and looking up a lot of things. But after a while you notice regularities, become familiar with the usual suspects, make some productive mistakes, and get better at it until it becomes easy and fun.
Another way mineral identification is like cooking is that professionals can go to school, learn to use expensive equipment and master the subject fully, yet amateurs can handle nearly all the common possibilities using just a few simple tools.
The first thing to do is to observe and test your mineral. (Review "What Is a Mineral?" for what exactly a mineral is.) Use the largest piece you can find, and if you have several pieces, make sure sure that they are all the same mineral. Examine your mineral for all of the following properties, writing down the answers. After that you'll be ready to take your information to the right place. Step 2: Luster
Luster is the way a mineral reflects light and the first key step in mineral identification. Look for luster on a fresh surface. The three major types of luster are metallic, glassy (vitreous) and dull. A luster between metallic and glassy is called adamantine, and a luster between glassy and dull is called resinous or waxy.
Step 3: Hardness
Use the 10-point Mohs hardness scale. The important hardnesses are between 2 and 7. For this you'll need your fingernail (hardness about 2), a coin (hardness 3), a knife or nail (hardness 5.5) and a few key minerals.
Step 4: Color
Color is important in mineral identification, but it can be a complicated subject. Experts use color all the time because they have learned the usual colors and the usual exceptions for common minerals. If you're a beginner, pay close attention to color but do not rely on it. First of all, be sure you aren't looking at a weathered or tarnished surface, and examine your specimen in good light.
Color is a fairly reliable indicator in the opaque and metallic minerals—for instance the blue of the opaque mineral lazurite or the brass-yellow of the metallic mineral pyrite.
In the translucent or transparent minerals, color is usually the result of a chemical impurity and should not be the only thing you use. For instance, pure quartz is clear or white, but quartz can have many other colors.
Try to be precise with color. Is it a pale or deep shade? Does it resemble the color of another common object, like bricks or blueberries? Is it even or mottled? Is there one pure color or a range of shades?
If you have an ultraviolet light, this is the time to see if the mineral has a fluorescent color. Make note if it displays any other special optical effects.
Step 5: Streak
Streak is the color of the finely crushed mineral. Streak is somewhat more reliable than color and is essential for a few minerals. You'll need a streak plate or something like it. A broken kitchen tile or even a handy sidewalk can do. Scratch your mineral across the streak plate with a scribbling motion.
Step 6: Crystal Form and Mineral Habit
A good knowledge of crystals is very helpful once you're past the beginner stage, but often minerals do not display any crystal faces, so for simplicity's sake we'll ignore it. For beginners, a mineral's crystal form is less important than its cleavage (see the next step). When you're ready to learn this aspect of minerals, you'll want a book.
One thing even beginners can do, though, is to observe a mineral's habit, the general form it takes. There are more than 20 different terms describing habit
Step 7: Cleavage and Fracture
Cleavage is the way a mineral breaks. Many minerals break along flat planes, or cleavages—some in only one direction (like mica), others in two directions (like feldspar), and some in three directions (like calcite) or more (like fluorite). Some minerals, like quartz, have no cleavage. Cleavage is a profound property that results from a mineral's molecular structure, and cleavage is present even when the mineral doesn't form good crystals. Cleavage can also be described as perfect, good or poor.
Fracture is breakage that is not flat. The two main kinds of fracture are conchoidal (shell-shaped, as in quartz) and uneven. Metallic minerals may have a hackly (jagged) fracture. A mineral may have good cleavage in one or two directions but fracture in another direction.
To determine cleavage and fracture, you'll need a rock hammer and a safe place to use it on minerals. A magnifier is also handy, but not required. Carefully break the mineral and observe the shapes and angles of the pieces. It may break in sheets (one cleavage), splinters or prisms (two cleavages), cubes or rhombs (three cleavages) or something else.
Step 8: Magnetism
Magnetism is a distinctive property in a few minerals. Magnetite is the prime example, but a few other minerals may be weakly attracted by a magnet, notably chromite (a black oxide) and pyrrhotite (a bronze sulfide). Use a strong magnet. The magnets I use came from the corners of an old plastic shower curtain. Another way to test magnetism is to see if the specimen attracts a compass needle.
Step 9: Other Mineral Properties
Taste is definitive for halite (rock salt), of course, but a few other evaporite minerals also have distinctive tastes. Just touch your tongue to a fresh face of the mineral and be ready to spit—after all it's called taste, not flavor. Don't worry about taste if you don't live in an area with these minerals.
Fizz means the effervescent reaction of certain carbonate minerals to the acid test. For this test, vinegar will do. (Learn more about the acid test)
Heft is how heavy a mineral feels in the hand, an informal sense of density. Most rocks are about three times as dense as water, that is, they have a specific gravity of about 3. Make note of a mineral that is noticeably light or heavy for its size.
Step 10: Look It Up
Now you are ready for mineral identification. Once you have observed and noted these mineral properties, you can take your information to a book or to an online resource. Start with my table of the rock-forming minerals, because these are the most common and the ones you should learn first. Each mineral's name is linked to a good photograph and notes to help you confirm the identification. If your mineral has metallic luster, go to my Minerals with Metallic Luster gallery to see the most likely minerals in this group. If your mineral is not one of these, try the sources in the Mineral Identification Guides category.

The Mohs scale consists of these ten standard minerals:
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Talc
11. Gypsum
12. Calcite
13. Fluorite
14. Apatite
15. Feldspar
16. Quartz
17. Topaz
18. Corundum
19. Diamond
The Mohs scale was devised by Friedrich Mohs in 1812 (and therefore it's never spelled "Moh's"). You use the Mohs scale by testing your unknown mineral against one of these standard minerals. Whichever one scratches the other is harder, and if both scratch each other they are both the same hardness.
The Mohs scale is strictly a relative scale, but that's all that anyone needs. In terms of absolute hardness, diamond (hardness 10) actually is 4 times harder than corundum (hardness 9) and 6 times harder than topaz (hardness 8). Because it isn't made for that kind of precision, the Mohs scale uses half-numbers for in-between hardnesses. For instance, dolomite, which scratches calcite but not fluorite, has a Mohs hardness of 3½ or 3.5.
There are a few handy objects that also fit in the Mohs scale. A fingernail is 2½, a penny (actually, any current U.S. coin) is just under 3, a knife blade is 5½, glass is 5½, and a good steel file is 6½. Common sandpaper uses artificial corundum and is hardness 9; garnet paper is 7½.
Mohs hardness is just one aspect of identifying minerals. Along with Mohs hardness, you need to consider luster, cleavage, crystalline form, color, and rock type to zero in on an exact identification

Weathering

Weathering is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity.
Two important classifications of weathering processes exist — physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals.[1]
The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil.
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Physical weathering
Physical weathering is the only process that causes the disintegration of rocks without chemically changing it. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process.

Thermal expansion
Thermal expansion, also known as exfoliation, insolation weathering or thermal shock, often occurs in areas, like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping greatly at night. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers. The stress causes the peeling off of the outer layers of rocks in thin sheets. Though this is caused mainly by temperature changes, thermal expansion is enhanced by the presence of moisture. Forest fires and range fires are also known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense, localized heat can rapidly expand a boulder, causing its surface to exfoliate or spall.
Frost disintegration
This process can also be called frost shattering, frost-wedging or freeze-thaw weathering. This type of weathering is common in mountain areas where the temperature is around the freezing point of water. Moist soils expand or frost heave upon freezing as a result of water migrating along from unfrozen areas via thin films to collect at growing ice lenses. This same phenomena occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up.[2] It is caused by the expansion of ice when water freezes, so putting considerable stress on the walls of containment. The same process acts on roads, creating potholes after the thaw.
Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point—that is, mainly alpine and periglacial areas. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor where it results in the formation of tors. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure.
[edit] Pressure release


Pressure Release of granite.
In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures. Pressure release is also known as "exfoliation" or "sheeting"; these processes result in batholiths and granite domes, an example of which is Dartmoor.
[edit] Hydraulic action
This is when water (generally from powerful waves) rushes into cracks in the rockface rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack itself.
[edit] Salt-crystal growth (haloclasty)


The surface pattern on this pedestal rock is honeycomb weathering, caused by salt crystallisation. This example is at Yehliu, Taiwan.


Salt weathering of building stone on the island of Gozo, Malta


Salt weathering of sandstone near Qobustan, Azerbaijan.
Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline (see salinity) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock.
Salt crystallization may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.
The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more.
It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honeycombed stones in sea wall. Honeycomb is a type of tafoni, a class of cavernous rock weathering structures, which likely develop in large part by chemical and physical salt weathering processes.
[edit] Biological Weathering
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes.
[edit] Chemical weathering


Comparison of unweathered (left) and weathered (right) limestone.
Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form. This is done through a combination of water and various chemicals to create an acid which directly breaks down the material.
Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation and hydrolysis are most important.
[edit] Dissolution
Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.
One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas[Citation Needed.]. Carbonation is therefore is a large feature of glacial weathering.
The reactions as follows:
CO2 + H2O -> H2CO3
carbon dioxide + water -> carbonic acid
H2CO3 + CaCO3 -> Ca(HCO3)2
carbonic acid + calcium carbonate -> calcium bicarbonate
Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.
[edit] Hydration
Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.
When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.


A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of sandstone was found in glacial drift near Angelica, New York
[edit] Hydrolysis and Silicate Weathering
Hydrolysis is a chemical weathering process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction:
Mg2SiO4 + 4H+ + 4OH- ⇌ 2Mg2+ + 4OH- + H4SiO4
olivine (forsterite) + four ionized water molecules ⇌ ions in solution + silicic acid in solution
This reaction results in complete dissolution of the original mineral, assuming enough water is available to drive the reaction. However, the above reaction is to a degree deceptive because pure water rarely acts as a H+ donor. Carbon dioxide, though, dissolves readily in water forming a weak acid and H+ donor.
Mg2SiO4 + 4CO2 + 4H2O ⇌ 2Mg2+ + 4HCO3- + H4SiO4
olivine (forsterite) + carbon dioxide + water ⇌ Magnesium and bicarbonate ions in solution + silicic acid in solution
This hydrolysis reaction is much more common. Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO2 in the atmosphere and can affect climate.
Aluminosilicates when subjected to the hydrolysis reaction produce a secondary mineral rather than simply releasing cations.
2KAlSi3O8 + 2H2CO3 + 9H2O ⇌ Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-
Orthoclase (aluminosilicate feldspar) + carbonic acid + water ⇌ Kaolinite (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution
[edit] Oxidation
Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. This process is better known as 'rusting'. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, such as chalcopyrites or CuFeS2 oxidizing to copper hydroxide and iron oxides.
[edit] Biological
A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. moss on roofs is classed as weathering.


Biological weathering of lava by lichen, La Palma.
The most common form of biological weathering is the release of chelating compounds, i.e. acids, by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.[citation needed] Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.
Building weathering
Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain.
Examples


Delicate Arch, Utah, USA


Alofaaga Blowholes


A rock formation in the Altiplano, Bolivia, sculpted by wind erosion
Spectacular examples of weathering include such features as natural arches and blowholes.

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