- Introduction
- What are minerals?
- How are igneous rocks classified?
- How are sedimentary rocks classified?
- How are metamorphic rocks classified?
- Open Source Web links
- Glossary terms
Introduction
The solid earth is made of rocks, which are made of minerals. To understand geology, you need to have some familiarity with rocks and minerals, what they are, how they are identified, and the geologic processes that create them.
To assist you in examining, identifying, and thinking about minerals and rocks, tables for identifying them are available to you. Links to the appropriate table and instructions for its use are located in each rock and mineral section. This basics page gives you the background needed to understand the terms used in the rock and mineral tables.
What are minerals?
All rocks except obsidian and coal are made of minerals. (Obsidian is a volcanic rock made of glass and coal is made of organic carbon.) Most rocks contain several minerals in a mixture characteristic of the particular rock type. When identifying a rock you must first identify the individual minerals that make up that rock.
Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystal lattice structure. Although thousands of minerals in the earth have been identified, just ten minerals make up most of the volume of the earth's crust--plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay.
Together, the chemical formula (the types and proportions of the chemical elements) and the crystal lattice (the geometry of how the atoms are arranged and bonded together) determine the physical properties of minerals.
The chemical formula and crystal lattice of a mineral can only be determined in a laboratory, but by examining a mineral and determining several of its physical properties, you can identify the mineral. First, you need to become familiar with the physical properties of minerals and how to recognize them.
What are the physical properties of minerals?
The physical properties of a mineral are controlled by its chemical composition (which types of atoms it consists of, and in what proportions) and its crystal lattice (the three-dimensional geometric pattern in which those atoms are arranged and bonded together).
It is no coincidence that crystals of quartz (SiO2) are six-sided, while crystals of halite (NaCl) are cubic. This is because of the geometry of their crystal lattices. It is also no coincidence that quartz is hard enough to scratch glass and will not dissolve in water to any visible extent, whereas halite will not scratch glass and will easily dissolve in water. These differences are due to the different chemical compositions of the minerals. The sodium (Na) and chlorine (Cl), by their chemical nature, readily break their bonds and become dissolved ions in water. The silicon (Si) and oxygen (O) in quartz are linked by strong bonds, which do not yield easily to the dissolving force of water.
Each mineral exhibits a unique set of physical properties. Therefore, the main task in identifying a mineral is to determine its physical properties. The physical properties that we will consider are color, luster, streak, cleavage, fracture, hardness, crystal shape and selected special properties.
Color Color is often useful, but should not be relied upon. Some minerals come in many different colors. Quartz, for example, may be clear, white, gray, brown, yellow, pink, red, or orange. So color can help, but do not rely on color as the determining property.
Luster Luster is how the surface of a mineral reflects light. It is not the same thing as color, so it crucial to distinguish luster from color. For example, a mineral described as "shiny yellow" is being described in terms of luster ("shiny") and color ("yellow"), which are two different physical properties. Standard names for luster include metallic, glassy, pearly, silky, greasy, and dull. It is often useful to first determine if a mineral has a metallic luster. A metallic luster means shiny like polished metal. For example cleaned polished pieces of chrome, steel, titanium, copper, and brass all exhibit metallic luster as do many other minerals. Of the nonmetallic lusters, glassy is the most common and means the surface of the mineral reflects light like glass. Pearly luster is important in identifying the feldspars, which are the most common type of mineral. Pearly luster refers to a subtle irridescence or color play in the reflected light, same way pearls reflect light. Silky means relecting light with a silk- like sheen. Greasy luster looks similar to the luster of solidified bacon grease. Minerals with dull luster reflect very little light. Identifying luster takes a little practice. Remember to distinguish luster from color.
Streak Streak is the color of the mineral as a powder. It is determined by scratching a mineral against a streak plate and checking the color of the streak left behind. The streak, the color of the mineral as a powder, may be different from the whole mineral color.
Cleavage A mineral that naturally breaks into perfectly flat surfaces is exhibiting cleavage. Not all minerals have cleavage. A cleavage represents a direction of weakness in the crystal lattice. Cleavage surfaces can be distinguished by how they consistently reflect light, as if polished, smooth, and even. The cleavage properties of a mineral are described in terms of the number of cleavages and, if more than one cleavage, the angeles between the cleavages. The number of cleavages is the number or directions in which the mineral cleaves. A mineral may exhibit 100 cleavage surfaces parallel to each other. Those represent a single cleavage because the surfaces are all oriented in the same diretion. The possible number of cleavages a mineral may have are 1,2,3,4, or 6. If more than 1 cleavage is present, and a device for measuring angles is not available, simply state whether the cleavages intersect at 90° or not 90°.
To see mineral cleavage, hold the mineral up beneath a strong light and move it around, move it around some more, to see how the different sides reflect light. A cleavage direction will show up as a smooth, shiny, evenly bright sheen of light reflected by one set of parallel surfaces on the mineral.
Fracture All minerals have fracture. Fracture is breakage, which occurs in directions that are not cleavage directions. Some minerals, such as quartz, have no cleavage whatsoever. When a mineral with no cleavage is broken apart by a hammer, it fractures in all directions. Quartz is said to exhibit conchoidal fracture. Conchoidal fracture is the way a thick piece of glass breaks with concentric, curving ridges on the broken surfaces. However, some quartz crystals have so many flaws that instead of exhibiting conchoidal fracture they simply exhibit irregular fracture. Irregular fracture is a standard term for fractures that do not exhibit any of the qualities of the other fracture types. In introductory geology, the key fracture types to remember are irregular, which most minerals exhibit, and conchoidal, seen in quartz.
HardnessHardness is the strength with which a mineral resists its surface being scraped or punctured. In working with hand samples without specialized tools, mineral hardness is specified by the Mohs hardness scale. The Mohs hardness scale is based 10 reference minerals, from talc the softest (Mohs hardness of 1), to diamond the hardest (Mohs hardness of 10). It is a relative, or nonlinear, scale. A hardness of 2.5 simply means that the mineral is harder than gypsum (Mohs hardness of 2) and softer than calcite (Mohs hardness of 3). To compare the hardness of two minerals see which mineral scratches the surface of the other.
Mohs Hardness Scale | |
---|---|
Index Minerals | Common Objects |
1-talc | |
2-gypsum | 2.5-fingernail |
3-calcite | 3.5-pure, untarnished copper |
4-flourite | |
5-feldspar | 5 to 5.5-stainless steel |
5.5 to 6-glass | |
6-apatite | 6 to 6.5-hard steel file |
7-quartz | |
8-topaz | |
9-corundum | |
10-diamond |
Crystal Shape All minerals are crystalline, but only some have the opportunity to exhibit their crystal forms. Many minerals in an introductory geology lab do not exhibit their crystal form. If a mineral has space while it grows, it may form natural crystals, with a crystal shape reflecting the geometry of the mineral's internal crystal lattice. The shape of a crystal follows the symmetry of its crystal lattice. Quartz, for instance, forms six-sided crystals, showing the hexagonal symmetry of its crystal lattice. There are two complicating factors to remember here: (1) minerals do not always form nice crystals when they grow, and (2) a crystal face is different from a cleavage surface. A crystal face forms during the growth of the mineral. A cleavage surface is formed when the mineral is broken.
Special Properties There are some properties that only help to distinguish a small number of minerals, or even just a single mineral. An example of such a special property is the effervescent reaction of calcite to a weak solution of hydrochloric acid (5% HCl). Calcite fizzes or effervesces as the HCl solution dissolves it and creates CO2 gas. Calcite is easy to identify even without testing the reaction to HCl, by its hardness, luster and cleavage.
Another special property is magnetism. This can be tested by seeing if a small magnet responds to the mineral. The most common mineral that is strongly magnetic is the mineral magnetite. A special property that shows up in some sample of plagioclase feldspar is its tendancy to exhibit striations on cleavage surfaces. Striations are perfectly straight, fine, parallel lines. Magnification may be required to see striations on plagioclase cleavage surfaces. Other special properties may be encountered on a mineral to mineral basis.
How to identify minerals
First, you need good light and a hand lens or magnifying glass. A hand lens is a small, double-lens magnifying glass that has a magnification power of at least 8X and can be purchased at some bookstores and nature stores.
Minerals are identified on the basis of their physical properties, which have been described in the the previous section. To identify a mineral, you look at it closely. At a glance, calcite and quartz look similar. Both are usually colorless, with a glassy luster. However, their other properties they are completely different. Quartz is much harder, hard enough to scratch glass. Calcite is soft, and will not scratch glass. Quartz has no mineral cleavage and fractures the same irregular way glass breaks. Calcite has three cleavage directions which meet at angles other than 90°, so it breaks into solid pieces with perfectly flat, smooth, shiny sides.
When identifying a mineral, you must:
- Look at it closely on all visible sides to see how it reflects light
- Test its hardness
- dentify its cleavage or fracture
- Name its luster
- Evaluate any other physical properties necessary to determine the mineral's identity
In the minerals tables that accompanies this section, the minerals are grouped according to their luster and color. They are also classified on the basis of their hardness and their cleavage or fracture. If you can identify several of these physical properties, you can identify the mineral.
Follow this link to the minerals classification table.
How are igneous rocks classified?
There are two main types of igneous rocks: (1) plutonic (intrusive) rocks, which form by solidification of molten rock deep within the earth, and (2) volcanic (extrusive) rocks, which solidify from molten rock erupted to the surface. Volcanic rocks break down into two more categories: (a) lava flows and (b) tephra (pyroclastic material).
Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions.[insert example as in sentence after next] The way that magma turns into a solid rock gives it a distinctive igneous texture. For example, magma that becomes a pluton by slowly crystallizing (growing minerals) within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash flowing across a landscape and then settling down and welding itself together into solid rock.
Igneous rock textures
The texture of an igneous rock results from the cooling, crystallization, and solidification history of the magma that formed it. Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive, lava flow or pyroclastic.
Texture in this context is not whether the rock feels rough or smooth to the touch. Igneous texture terms have objective definitions that refer only to igneous rocks.
Volcanic rocks
Let us start with textures associated with rocks formed by lava flows. Magmas that erupt as lava onto the earth's surface cool and solidify rapidly. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture.
Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix. This texture is called porphyritic.
If lava has bubbles of gas escaping from it as it solidifies, it will end up with "frozen bubble holes" in it. These "frozen bubble holes" are called vesicles, and the texture of a rock containing them is said to be vesicular.
If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture.
If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals (glass by definition is not a mineral, because it does not have a crystal lattice). Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black.
Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. However, instead of originating from lava that flowed on the earth's surface, tephra is volcanic material that was hurled through the air during a volcanic eruption.
A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in. The term "fine-grained, fragmental" is easy to confuse with the term fine-grained (aphanitic). An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff.
A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture. However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. The bigger chunks of material in a volcanic breccia are more than 1 cm (5/8 inch) across, and sometimes are much bigger.
Plutonic rocks
When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture.
Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. This hydrothermal fluid gets into cracks and voids in the earth's crust, and as it cools it may grow very large minerals from the dissolved chemical elements. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter (and sometimes is much larger). The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite.
Igneous rock compositions
The most common igneous compositions can be summarized in three words: mafic (basaltic), intermediate (andesitic), and felsic (granitic).
Felsic composition is higher in silica (SiO2) and low in iron (Fe) and magnesium (Mg). Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions.
Composition and color
Composition influences the color of igneous rocks. Felsic rocks tend to be light in color (white, pink, tan, light brown, light gray). Mafic rocks tend to be dark in color (black, very dark brown, very dark gray, dark green mixed with black). The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors (green, gray, brown).
The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition. If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic.
This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly.
The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. Geologists working in the field carry a rock hammer, so they can break off the weathered, outer parts of rocks to see the "fresh," unweathered rock inside.
If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. Igneous rocks with quartz in them are usually felsic. Igneous rocks with olivine in them are usually mafic. Igneous rocks with neither quartz nor olivine in them are most commonly intermediate.
Origins of igneous rocks
Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed. For example, a coarse-grained, felsic igneous rock is not only a granite, it is an intrusive igneous rock that formed from slow cooling and crystallization of a body of magma within the earth's crust.
How to identify igneous rocks
Igneous rocks can be distinguished from sedimentary rocks by the lack of beds, fossils, and rounded grains in igneous rocks, and the presence of igneous textures. A granite, for example, can be distinguished from a sandstone because rather than being a mixture of rounded clastic grains compressed and cemented together, granite consists of a small number of minerals in shiny black, white, or pink colors, with excellent crystal forms, grown together into a completely interlocking pattern. Sandstones, by contrast, have sedimentary bedding (layers) and consist of rounded grains with some spaces between the grains, which you can see with a hand lens or magnifying glass.
Igneous rocks can be distinguished from metamorphic rocks by the lack of foliation (layering) in the igneous rocks. The common metamorphic rocks that lack foliation, quartzite and marble, consist entirely of quartz grains and calcite, respectively. No igneous rock consists entirely of quartz grains or calcite.
Granite may look like gneiss at first glance, but granite has no layering, no preferred orientation of the minerals. The minerals in a granite grow randomly in all directions, rather than tending to grow parallel to each other.
Igneous rocks are classified on the basis of their texture and their composition. See the previous sections for descriptions of the different igneous textures and compositions.
The igneous rock classification tables that accompany this section are arranged on the basis of igneous textures first, and further broken down on the basis of igneous composition. Remember that igneous composition is estimated on the basis of color: light = felsic composition, medium = intermediate composition, and dark = mafic composition.
Follow this link to the igenous rock classification table.
How are sedimentary rocks classified?
There are two main groups of sedimentary rocks: chemical and clastic (clastic is sometimes called detrital). Each type of sedimentary rock is formed when sediments turn into rock. Chemical sediments are sediments that precipitate from solution, for example salt crystals that grow at the bottom of an evaporating body of water. Clastic sediments are solid pieces of weathered and eroded rocks or minerals, for example sand on a beach.
Sedimentary rock textures
Sedimentary rocks are made of lithified sediment. The texture of a particular sedimentary rock results from how the sediment was formed, where it was deposited, and how it became rock.
Sedimentary rock textures different from igneous rock textures. Sedimentary texture names only apply to sedimentary rocks.
Clastic textures
Clastic sedimentary textures are described in terms of the size of the sediment grains, how round they are, and how well they are sorted.
Grain size--The diameter or width of a clastic sediment grain determines its grain size. The words boulder, cobble, pebble, sand, silt, and clay refer to specific ranges of grain size. Sand grains, for example, are between 1/16 mm and 2 mm in diameter.
Rounding--Clastic sediment grains can be round, angular, or in-between (subangular or subrounded). Breccia is a clastic sedimentary rock that by definition consists largely of angular grains of pebble size or larger. Conglomerate, another sedimentary rock, consists largely of rounded grains of pebble size or larger.
Sorting-- The extent to which all the grains are the same size is known as sorting. If all the grains are the same size, they are well sorted. Some sandstones are well sorted, and some are not. Most conglomerates are poorly sorted, and consist of a mixture of grain sizes ranging from sand to pebble.
Chemical textures
Chemical sedimentary rocks, such as limestone, do not have a widely agreed-upon texture scheme, so we will not be concerned with the textures of chemical sedimentary rocks.
Sedimentary rock compositions
The composition of a sedimentary rock is defined on the basis of the minerals that compose the rock.
Some sandstones are made almost entirely of quartz. Sandstone made virtually entirely of quartz is called arenite.
Other sandstones have lots of feldspar mixed with quartz. Such sandstones are called arkose.
Other sandstones are a mixture of feldspar, quartz, clay, and small fragments of volcanic rock. Such sandstones are technically known as lithic wackes, although geologists often call them by their old name, graywackes, and the rocks are sometimes informally described as "dirty sandstones."
A clastic sedimentary rock simply must be made of sand-sized grains to qualify as sandstone, regardless of what minerals the grains are made of.
The composition of a chemical sedimentary rock is usually implied by the name of the rock. For example, all limestones consist mostly of the mineral calcite. Coal is made of carbon. Rock salt is made of salt minerals such as halite. Gypsum rock is made of the mineral gypsum. Chalcedony is made of microcrystalline quartz, quartz grains so tiny that they cannot be distinguished even with a standard optical microscope.
How to identify sedimentary rocks
The sedimentary nature of a rock is usually based on the presence of sedimentary beds. The presence of fossils also indicates that a rock is sedimentary. Clastic sedimentary rocks contain clastic grains, so a rock that looks like it consists of gravel, sand, or mud that has been turned into rock is probably a clastic sedimentary rock. Each type of chemical sedimentary rock has its own characteristics and these must be learned one-by-one. Coal, for example, consists of soft, black carbon. Limestone consists of the mineral calcite, which is too soft to scratch glass and fizzes in weak acid.
In using the sedimentary rock classification table that accompanies this section, you will see that the clastic sedimentary rocks are classified on the basis of grain size. Sandstones are rocks made of sand-size grains. Shale is layered sedimentary rock made of fine mud-size grains too small to see with the naked eye.
The chemical sedimentary rocks are classified on the basis of what they are made of. Rock salt is made of salt. Limestone is made of calcite. Coal is made of carbon.
Follow this link to the sedimentary rock classification table.
How are metamorphic rocks classified?
Metamorphic rocks form from pre-existing rocks that are metamorphosed into new rocks due to changes in temperature, pressure, and chemistry. The pre-existing rocks are called the parent rocks or protoliths. As rock is metamorphosed, new minerals form from the pre-existing minerals.
There is a rare type of metamorphic rock that apparently undergoes partial melting during metamorphism. Such rock is called migmatite, which means a mixture of igneous and metamorphic together in one rock. For all the other types of metamorphic rock discussed here the rocks remain in the solid state during metamorphism, without any melting taking place.
There are two main groups of metamorphic rocks, regional metamorphic rocks and contact metamorphic rocks. Regional metamorphic rocks form in zones where large volumes of the crust are subjected to heat and tectonic stress. Contact metamorphic rocks form next to igneous intrusions as a result of the heat coming from the magma.
There are two steps to classifying metamorphic rocks. The first step is to identify the rock on the basis of texture and foliation (or lack of foliation). The second step is to specify the minerals that are most visible or most noteworthy.
Foliated metamorphic rocks (regional metamorphism)
Foliated rocks originate during regional metamorphism. There are several specific types of foliation:
- Slaty cleavage. Slaty cleavage is a rock cleavage, not a mineral cleavage, and refers to the tendency of a very fine-grained rock to split into perfectly flat layers.
- Phyllitic foliation. Phyllitic foliations do not split apart into perfectly flat surfaces; instead, the foliation surfaces are slightly wrinkled. Phyllitic foliation surfaces have a shiny luster, even though the minerals in the rock are too small for the crystals to be discerned with the naked eye.
- Schistose foliation. Schistose foliation comes from mica minerals or other flat minerals that have grown large enough to be seen with the naked eye and are oriented parallel to each other. Micas are flat minerals that cleave apart into flexible sheets; examples include muscovite and biotite.
- Gneissic foliation. Gneissic foliation refers to layers that consist of different minerals, such as layers of black biotite and amphibole alternating with layers of clear quartz and white feldspar. This gives the rock a striped appearance. Minerals in a rock with gneissic foliation are generally large enough to for the crystals to be seen with the naked eye.
Foliated metamorphic rocks are named primarily on the basis of their foliation; hence, the four common types of regional metamorphic rock are slate, phyllite, schist, and gneiss.
This sequence-slate, phyllite, schist, and gneiss-is also a sequence of increasing metamorphic grade. Higher metamorphic grade occurs at higher temperature. Gneiss forms at much higher temperature than slate, and at somewhat higher temperature than schist.
In naming a foliated metamorphic rock, the prominent minerals should also be mentioned. For instance, if a rock has schistose foliation because of the mineral biotite, and the foliation surfaces are dotted here and there with red garnets, the rock should be called garnet biotite schist.
Unfoliated metamorphic rocks (contact and regional metamorphism)
Hornfels: Fine-grained, unfoliated metamorphic rocks that result from contact metamorphism are called hornfels.
Quartzite: Metamorphic rocks made entirely or almost entirely of quartz are called quartzite. Even if a quartzite forms during regional metamorphism, it may not be foliated. Quartzite can be distinguished from a quartz sandstone by the way the quartz grains have recrystallized and filled in around each other, eliminating the empty spaces between mineral grains that are present in clastic sedimentary rocks. The recrystallization makes quartzite a denser, tougher rock that breaks right through the quartz grains, rather than around them.
Marble: A rock made of calcite or dolomite recrystallizes into marble when it is metamorphosed. The marble may or may not be foliated (layered).
How to identify metamorphic rocks
Most metamorphic rocks are foliated, and are made of minerals in well-formed crystals that would not occur in sedimentary rocks. The way the minerals are oriented, such as flat minerals lying parallel to each other, creates the metamorphic foliation.
Gneiss may look like granite. However, the minerals in a gneiss are at least partly lined up parallel to each other, unlike the random orientation of minerals in granite.
The common metamorphic rocks that have no foliation, quartzite and marble, are made of virtually pure quartz grains or virtually pure calcite, respectively.
In using the metamorphic rock classification tables that accompany this section you will see that they are grouped according to whether the rock is foliated or not. Foliated metamorphic rocks are distinguished on the basis of the size of their mineral crystals and the exact type of foliation they have, as described in the section on foliation above. In the sequence of metamorphism from low-grade to high-grade metamorphism, the rocks proceed through the sequence slate-phyllite-schist-gneiss. Proceeding through this sequence the mineral crystals start too fine-grained to see in slate and become increasingly large and easy to see in schist and gneiss, and the foliation that starts out flat and smooth in slate becomes increasingly rough and uneven in schist and gneiss.
Follow this link to the metamorphic rock classification table.
Open Source Web Links
For pictures of the common minerals, go the U.S. Geological Survey Web page: http://geomaps.wr.usgs.gov/parks/rxmin/mineral.html
For pictures of common rocktypes, go to the U.S. Geological Survey Web page: http://geomaps.wr.usgs.gov/parks/rxmin/rock.html
Glossary terms that appear on this page: crystal lattice; luster; streak; cleavage; hardness; igneous; plutonic; volcanic; tephra; pyroclastic; aphanitic; porphyritic; vesicular; frothy; pumice; glassy; obsidian; tuffaceous; tuff; brecciated; phaneritic; hydrothermal; pegmatitic; pegmatite; lithify; arenite; arkose; graywacke; intrusion; foliation; slate; phyllite; schist; gneiss
Basics--Rocks & Minerals
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 3/30/11