Last modified 01 / 12

GeoMan's Mineral and Rock Glossary


Metallic | H<2.5 | H 2.5 to 3.5 | H 3.5 to 5.5 | H >5.5 | Glossary | Tests | Index

Rock Summary | Igneous | Sedimentary | Metamorphic


Background Information

In simple terms, our planet is just a big ball of very hot stuff which, at the surface where it is exposed to space, has cooled to form crystalline solids (igneous rocks) composed of minerals. Minerals in turn are combinations of elements, which are made of smaller particles, which are made of smaller particles, which are made of..... who knows what.

To be classified as a mineral, a substance must be an inorganic, naturally formed solid, with a specific chemical formula and a fixed internal structure. For example, coal is not a mineral (it's organic), but snow meets all five (5) requirements and therefore is a mineral.

For additional information:

Community College level

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High School level

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Definition of terms as related to the fundamental organization of matter

Atom: Composed of protons, electrons and neutrons

Crystallization: Liquid to solid phase change

Density: Mass divided by volume

Electron: Negatively charged particles surrounding the nucleus of an atom. Has an atomic mass of nearly zero (0).

Elements: Definable substances composed of atoms

Magma: Liquid rock

Mass: The amount of stuff

Matter: Anything which has mass and occupies space

Minerals: Solid matter composed of elements in specific combinations and arrangements

Neutron: Non-charged particle found in the nucleus of an atom. Has an atomic mass of one (1).

Phase change: Based on temperature (and pressure) all matter can exist in one of 4 separate phases - solid, liquid, vapor, or plasma

Proton: Positively charged particle found in the nucleus of an atom. Has an atomic mass of one (1).

Rocks: Composed of minerals in specific combinations

Sub-atomic particles: Fundamental building block of matter, including quarks and such

Volume: How much space is occupied by a mass


To summarize solid matter


-- is made of --


-- which are made of --


-- which are made of --


-- which are made of --


-- which are made of --

protons, neutrons, & electrons

-- which are made of --

subatomic particles

-- which are made of --

who knows what


Mineral and Rock Glossary

Angular: Any clast with sharp edges. Usually indicates recent mechanical fracturing.

Anhedral: No visible external crystal form. Unfortunately, most mineral specimens are anhedral.

Breakage Pattern: How a mineral breaks. See Cleavage and Fracture.

Clast: Any fragment of rock debris, usually weathered from bedrock by mechanical processes.

Clay: Very fine grained clasts, often the result of chemical weathering. Can be sub-divided into two (2) separate varieties, which is important when trying to assign field names to the fine-grained clastic sedimentary rocks:

Clay the Mineral: There are many varieties of Clay the Mineral, but all share several characteristics. They are the product of chemical weathering, are very small (microscopic), and have basal cleavage. The cleavage is important: Toss a deck of cards into the air and record how many land on the edges. Try it as many time as needed to convince yourself that the recorded number remains essentially zero (0). Well, each clay mineral is like a tiny card, and when they settle to the bottom of the water, they pile up in sub-parallel layers. Sedimentary rocks made from Clay the Mineral therefore have a distinct foliation, and are usually called claystone or shale.

Clay the Size: Any fine grained clast, but not necessarily with basal cleavage. They can be any shape: cubic, spherical, oblong, rounded, angular, whatever. Therefore, the sedimentary rocks made from Clay the Size may not exhibit foliation, and usually called siltstone or mudstone.

Cleavage: The ability of a mineral to split along parallel lines of weakness when broken, resulting in smooth, mirror-like breaks. Minerals can have 1, 2, 3, 4, or 6 directions of cleavage. If more than one (1) direction is present, the angle between the cleavage planes is an important diagnostic feature (such as "2 directions at 90°"). Cleavage is further defined based upon its quality, with such modifiers as "perfect," "good," and "poor" being used. The following are commonly accepted terms:

Basal cleavage: One (1) direction (ex. mica, clay, graphite)

Cubic cleavage: Three (3) directions at 90° (ex. galena, halite)

Color banding: Alternating layers of felsic (light colored) and mafic (dark colored) minerals common in high grade metamorphic rocks.

Composition: What something is made of. In igneous rocks, composition refers to the chemical characteristics of the parent magma and resulting rock. The terms ultramafic, mafic, intermediate, and felsic are commonly used terms. Composition is often related to plate tectonics.

Conchoidal fracture: Smooth, scalloped-shaped fractures common to quartz and other silica-rich minerals and rocks (as well as plate glass).

Crust: The upper surface of the planet which has cooled and crystallized where exposed to the coldness of space. The vast majority is composed of silicate minerals. However, the vast majority is also covered by a thin veneer of sedimentary debris and rock, successfully obscuring the igneous material lurking beneath.

Crystal: Common term used to indicate any mineral which grew into open space and therefore was free to take its perfect external form. Three (3) terms are used to indicate the degree of crystal development.

Euhedral: A perfect external crystal form with all faces developed.

Subhedral: Some external crystal form is visible.

Anhedral: No visible external crystal form. Unfortunately, most mineral specimens are anhedral.

Crystal habit: Six possible crystal systems are possible, and are based on the internal organization of the atoms.

Crystalline structure: The external crystal form of a mineral (if visible) is a reflection of the internal arrangements of the atoms. This arrangement is one of the fundamental defining properties of a mineral, and is always constant at the atomic level (see Crystal Habit). The external form (forming a "crystal") is only evident if the mineral was allowed to crystallize in open space.

Crystallization: The liquid to solid phase change which occurs when a magma loses energy.

Euhedral: A perfect external crystal form with all of its faces developed.

Extrusive: Igneous rocks which cool at the surface of the earth. Commonly exhibit fine grained textures. The most common extrusive igneous rock is basalt.

Feldspar: A common family of silicate minerals. The feldspars form approximately 60% of the earth's crust.

Felsic: A magma or rock relatively higher in silicon and oxygen (together called "silica"), aluminum, potassium.

Field: Term used to indicate the real world, outdoors, where the minerals and rocks live. Spend as much time there as possible (kill your TV).

Field names: There are two (2) ways to name a rock: macroscopic observation in the field, and by using a more detailed chemical and optical examination in a laboratory. I'm a field name kind of guy.

Fizz test: Minerals containing calcium carbonate (CaCO3) will generally react when exposed to any weak acid (usually hydrochloric acid (HCl), but even vinegar will work). Carbon dioxide (CO2) is released and the mineral or rock literally "fizzes." It is often necessary to scratch the rock (most, if not all, carbonates are relative soft) and make a small pile of powdered material (this increases the surface area).

Foliation: A visible layering within a sedimentary or metamorphic rock, usually the result of the parallel orientation of flat, platy minerals with basal cleavage. These can include Clay the Mineral (as in shale), or the various micas (in metamorphic rocks).

Fossil: Any evidence of life preserved in a rock (usually a sedimentary rock). Bones, teeth, shells, and footprints are all considered fossils.

Fossiliferous: Any rock (usually sedimentary) which contains a large proportion of fossils.

Fracture: In minerals, any specimen which does not exhibit cleavage when broken.

Conchoidal Fracture: Smooth, scalloped-shaped fractures common to quartz and other silica-rich minerals and rocks.

Frothy: An igneous texture common to extrusive igneous rocks. The frothy texture is the result of air bubbles trapped inside the cooling magma.

Glassy: An igneous texture which occurs when cooling is so rapid that there is no mineral growth and the resulting solid is a natural glass. Obsidian is an example.

Igneous rock: Cooled from magma. Igneous rocks form the bulk of the earth's crust.

Intrusive: Igneous rocks which cool inside the earth. Commonly exhibit medium to coarse grained textures. A common intrusive igneous rock is granite.

Lava: Magma which has erupted at the surface of the earth.

Macroscopic: Observations made without the use of high-powered magnification. Usually all we have in the field.

Mafic: A magma or rock higher in iron and magnesium, and relatively lower in silicon and oxygen.

Magma: The liquid form of rock.

Metamorphic rock: Any rock whose mineralogy has changed due to changing environmental conditions. Metamorphic processes are commonly the result of increases in heat and/or pressure, with the resulting changes in mineralogy always reflecting an attempt by the elements in the rock to regain equilibrium with the new conditions.

Mica: A low-temperature mineral family with basal cleavage. Common in felsic igneous and low grade metamorphic rocks. Common varieties include biotite (the mafic mica) and muscovite (the felsic variety).

Microscopic: Observations made with the use of high-powered magnification. Often used to refine field names derived from macroscopic observations made in the field.

Rounded: Any clast with smoothed edges. Usually indicates abrasion by moving water or wind.

Sedimentary rock: Secondary rocks formed from the accumulation of sediments, chemical precipitates, and/or biological residue.

Silicates: Minerals based on the SiO4 tetrahedron. The vast majority of the earth's crust is igneous rock, and silicates are the most common mineral class in igneous rocks.

SiO4 tetrahedron: Basic building block of the crust. One (1) atom of silicon combined with four (4) atoms of oxygen. Oxygen composes 46.6% of the crust by weight (and nearly 94% by volume), with silicon accounting for an additional 25% by weight. The crust is essentially a boxwork of oxygen atoms joined together by silicon, and a few other elements.

Subhedral: Some external crystal form is visible.

Texture: In igneous rocks, texture relates to the size of individual mineral grains, and is usually the result of cooling history.

Tink test: A simple field method of distinguishing between shale (a sedimentary rock composed of Clay the Mineral) and slate (a metamorphic rock composed of interlocking grains of minerals with basal cleavage). Shale, being composed of piles of loose debris, goes "thunk" when dropped, while slate, being harder and more tightly inter-grown, goes "tink." Try it and you'll see.

Ultramafic: More mafic than mafic. Ultramafic igneous rocks are composed almost entirely of olivine and pyroxene, with no feldspar. The ultramafic are thought to originate within the upper mantle of the earth, and are very rare at the surface.


Metallic | H<2.5 | H 2.5 to 3.5 | H 3.5 to 5.5 | H >5.5 | Glossary | Tests | Index

Rock Summary | Igneous | Sedimentary | Metamorphic


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