Earth Materials I

Crystallography & Mineralogy

Introduction

Our physical environment is composed of solid, liquid and gaseous substances. These in turn are composed of matter that has both material content and structure. For earth materials, there is a basic hierarchy of composition/structure that proceeds from atoms that comprise the various elements - compounds (combinations of elements chemically bound together) - minerals - rocks.

ITEM		DEFINITION
	Elements (atoms)	Basic unit of matter with consistent chemical properties
	Compounds	Mixtures of one or more elements that are held together by chemical bonds
	Minerals	Naturally occurring compounds with a specific composition composed of atoms with a regular ordered arrangement (crystalline structure)
	Rocks	Aggregates of minerals.

In this exercise, we will focus on minerals - these are the basic units of solid (rocky or stony) matter that make up our planet and many of the "smaller" objects in the Solar System.

Mineral Classifications

There are some 3500+ known minerals. These have been classified into different groups, called mineral classes that are related principally on the basis of similar composition.

Mineral Class	Defining Ion	Mineral example	Composition
Silicates	Silicate ion (SiO4^4-)	Olivine	(Mg, Fe)SiO4
Carbonates	Carbonate ion (CO3^2-)	Calcite	CaCO3
Oxides	Oxygen ion (O^2-)	HÊmatite	Fe2O3
Halides	Cl^-, F^-, Br^-, I^-	Halite (salt)	NaCl
Sulfates	Sulfate ion (SO4^2-)	Anhydrite	CaSO4
Sulfides	S^2-	Pyrite	FeS2
Native Elements	the element itself	Native Copper	Cu

Silicates 

Silicates are minerals whose structure is based on the silicate tetrahedron, SiO₄. This consists of a Silicon atom (Si) bound to 4 Oxygen atoms, evenly distributed in space around the Si center, which results in a tetrahedron structure.

These tetrahedra can be arranged or linked together in a variety of ways and it is this arrangement that largely determines the underlying crystalline structure of the silicate minerals. Typically, the underlying structure is defined by the nature of common Oxygen atoms that are shared by adjacent tetrahedra. For example, of each tetrahedron shares 2 Oxygen atoms, a "chain" of tetrahedra is formed:

Silicate Class	Tetrahedral arrangement	model
Orthosilicates	Isolated tetrahedra
Cyclosilicates	Isolated rings
Inosilicates	Single or double chains
Phyllosilicates	Sheets or layers
Tectosilicates	Three-Dimensional linkage
Sources: These images are from www.mineralworld.co.uk

Common Minerals

Most minerals are actually somewhat rare, and so most geologists can get by with recognizing only a handful of all the known types. We have assembled a partial list of common mineral types that are relevant to discussions in class. These include rock-forming minerals that are important to the construction of the Earth and the other terrestrial planets, in addition to minerals important in biology (biomineralization).

Name: Quartz

Formula; SiO₂

Silicate class: Tectosilicate

Comments: Quartz comes in many different forms, ranging from large individual crystals visible to the naked eye to microscopic crystals. Crypto-crystalline quartz forms individual grains too small to seen easily, even with a microscope. This form of quartz is called chert (aka flint). A form that is microscopically fibrous is called chalcedony. Opal is amorphous quartz (i.e. the silica groups are not aligned in any regular arrangement that contains interstitial water molecules. It is usually designated, SiO₂ÄnH₂O. Diatoms use opal to form their outer casings, or frustrules. The discarded frustrules can make significant contributions to bottom sediments in the oceans, diatomaceous earth, and are the principal "grit" found in toothpaste.

Name: Calcite

Formula; CaCO₃

Mineral class: Carbonate

Comments: Calcite is often distinguishable from other minerals based on its rhombohedral shape. Calcite is not only common, but also widespread. It is found in many sedimentary rocks, as a cementing agent, or in fossils. Calcite is used industrially in making portland cement, mortar, pharmaceuticals, and is a major constituent of marble.

Name: Pyrite

Formula; FeS₂

Mineral class: Sulfide

Comments: Pyrite forms under reducing (low Oxygen) conditions. When oxygenated water comes in contact with pyrite, the water oxidizes and forms sulfuric acid. Pyrite is often found in igneous rocks, although it can be found in metamorphic rocks and the sedimentary rock, coal.

Name: Gypsum/anhydrite

Formula; CaSO₄Ä2H₂O / CaSO₄

Mineral class: Sulfide

Comments: Gypsum is very common in marine evaporite deposits. Less commonly, gypsum is found as a precipitate from saline lakes. Gypsum is sometimes formed by adding water to the primary anhydrite. The use of gypsum goes back approximately 5000yrs. Usually, gypsum is mined for making wallboard (drywall) and/or portland cement.

Name: Hematite

Formula; Fe₃O₄

Mineral class: Oxide

Comments: Hematite is related to a suite of iron-oxides which have similar formulas but represent different mineral types. In addition, hematite itself can take on many different macroscopic forms. Magnetite has the chemical formula (Fe₃O₄), but occurs with an octagonal crystalline structure. As its name implies, magnetite is magnetic.

Name: Siderite

Formula; FeCO₃

Mineral class: Carbonate

Comments: Siderite is commonly distinguished from other rhombohedral carbonates, like calcite, by its color and high specific gravity, or heavy weight. Siderite is commonly found in sedimentary rocks or in metamorphosed iron formations. Siderites known uses include; used as an iron ore, for pigments used in paints, cosmetics, and other materials in which a red or brown color is desired.

Name: Olivine

Formula: (Mg,Fe)₂SiO₄

Mineral Class: Orthosilicate

Comments: Olivine is a common mineral in mafic (45-55% SiO₂) and ultramafic (<45% SiO₂) igneous rocks. These rocks are usually associated with calcium-rich minerals. Mafic and ultramafic minerals, especially olivine, usually crystallize at much higher temperatures than most other minerals. Crystallization of olivine usually occurs between 1000-1200∞ C. Olivine is most commonly olive to yellowish green in color. The clear gem variety, peridote, is used as refractory sand used for making molds during certain types of casting procedures.

Name: Biotite

Formula: K(Mg, Fe)₃(AlSi₃O₁₀)(OH)₂

Mineral Class: Sheet silicate

Comments: Sheet silicate mineral form control sheet-like fragmentation. Biotite is very common in igneous rocks such as granite, granidiorite, pegmatite, etc. Biotite is also common in metamorphic rocks. When hydrothermal processes are enacted upon biotite, its new puffed-up alteration product, vermiculite, is usually used in insulation, gypsum wallboards, and/or household potting soil.

Name: Muscovite

Formula: KAl₂(AlSi₃O₁₀)(OH)₂

Mineral Class: Sheet silicate

Comments: Muscovite, like biotite, has sheet-like fragmentation. Muscovite is a common constituent in granitic pegmatite, granite, aplite, etc. Muscovite is also common in metamorphic rocks. An early use for musocovite was as a substitute glass because the thin sheets are relatively transparent. More currently, it is used in electronics and industrial applications.

Name: Potassium Feldspar (aka K-Spar)

Formula: KalSi₃O₈

Mineral Class: Framework silicate

Comments: K-Spar actually has four different varieties: Microcline, Orthoclase, Low Sanidine, and High Sanidine. It is used in making glass and also manufacturing ceramics such as, floor tiles and dinnerware.

Name: Amphibole

Formula: (Mg,Fe)₂(Mg,Fe)₃Si₈O₂₂(OH)₂

Mineral Class: Chain Silicates

Comments: Amphibole is commonly brown, brownish green, or white. The name amphibole is actually a generic term for the entire amphibole group. Each member of the group varies by certain physical and optical characteristics. Fibrous varieties of amphiboles are sometimes used as asbestos, although the use is limited. The health problem linked to asbestos are from fibrous anthopyllite, but the rock anthophyllite will not cause any harm- just donπt grind it up and take a deep breath.

Name: Halite

Formula: NaCl

Mineral Class: Halide

Comments: Crystals of halite are cubic in shape. Halite is commonly found in marine evaporate deposits that can form beds over a thousand meters thick. Halite, also known as table salt, has been used for centuries usually to add flavor to food or to preserve meet since iceboxes did not yet exist. Other uses include leather tanning and treatment, metal processing, pharmaceuticals, and to melt ice off roads and sidewalks. Halite is generally mined or saline water is pumped into a shallow pond where it can evaporate. The resulting salt brine from evaporation is then pumped to the surface for industrial and domestic uses.

Table 1. In a mineral, the different kinds of atoms and the way they bond will affect the way light will pass through that mineral; this is how luster is controlled in every mineral.

MINERAL	LUSTER
Metallic	Strong reflection
Vitreous	Bright like glass
Pearly	Iridescence of a pearl
Adamantine	Luster of a diamond
Earthy	Feels like dirt

MINERAL IDENTIFICATION

Directions

Using the above information, the table of mineral luster, and your own inspection of the hand samples to fill in each column for each mineral as best you can. Feel free to form groups, talk to other class members, the teacher, and other resources available in the classroom. Some information has been provided for you and is already filled in on the sheets. Lab worksheets are due next week.

Head start for next weeks lab (aka- homework)

You will notice on the syllabus, there are some, seemingly, misspelled words usually found in parentheses. They are actually the names of some different rock types. For lab next week, please look up some information on these rocks similar to the information that was provided for the minerals this week. You might include, but are not limited to, information about where and how the rock is formed, common minerals found in the rock, and common uses for the rock.

References:

Levin, H.L., 1999, The Earth Through Time (6^th Ed); Saunders College Publishing, Florida, 568p.

Nesse, W.D., 2000, Introduction to Mineralogy; Oxford University Press, New York, 442p.

Press, F. & Siever, R., 2000, Understanding Earth (3^rd Ed); W.H. Freeman & Company, USA, 121p.