Definition. Minerals are naturally-occurring chemical substances, uniform in terms of composition, generally crystalline, and, with few exceptions, of a solid consistency. The fact that they are naturally formed is of great significance. Substances created by the hand of man - whether it be crystal glass or crystalline silicon or artificial diamonds - is a mineral, even though it might fit the definition perfectly. There are presently 3,840 individually identified and named minerals.
Crystals. Crystals are substances which are solid and whose constituent atoms are arranged in a repetitive, three-dimensional pattern which is unique for each mineral type. The basic architectural unit of a crystal is the unit cell which consists of a specific number atoms organized in a specific spatial arrangement which is unique for that particular mineral. As a rule, crystals are bounded by symmetrically arranged planar surfaces called faces (or hedra) which can also be used as an identification criterion. Crystals may be naturally-occuring or artificially produced (synthetic). Not all crystalline objects possess faces. This is due to the fact that in order to develop faces there must be ample room for a single crystal to grow unimpeded by the presence of neighboring crystals. This is situation is relatively rarely observed in nature. The terms anhedral, subhedral and euhedral refer to crystals that possess no faces, some faces, and all faces respectively. Minerals whose atoms are not arranged regularly in the form of a unit cell are called amorphous.
Crystal-Chemical Classification. Minerals are substances comprised of one or more of the 92 naturally-occuring chemical elements. Scientists who study the properties of minerals, called mineralogists, assign minerals to groups on the basis of their chemical composition and structure. This system of classification is used also by mineral collectors who wish to arrange their collection systematically. However this method is not really useful for simple mineral identification purposes, since very complex and expensive chemical analytical methods and tools are required to make determinations of a mineral's identity. There are eight individual crystal-chemical groups and these are:
Elements: minerals that consist of a single element, such as sulfur (S), graphite (C), diamond (C), and gold (Au).
Sulfides: minerals that are compounds of sulfur with other elements (except oxygen), such as pyrite (FeS2), galena (PbS), sphalerite (ZnS), and arsenopyrite (FeAsS)
Halides: minerals that are compounds of metals with the halogens fluorine, chlorine, bromine, or iodine, such as fluorite (CaF2) and halite (NaCl).
Oxides: minerals that are compounds of elements with oxygen, such as quartz (SiO2), rutile (TiO2), and magnetite (Fe2O3). Also in this category are hydroxides which are compounds of elements with the hydroxyl group - OH - such as goethite (FeO(OH)).
Sulfates: minerals that are compounds of elements with the sulfate group - SO4 - such as barite (BaSO4), and anhydrite (CaSO4). Also in this category are chromates such as crocoite (PbCrO4), molybdates such as wulfenite (PbMoO4), and tungstates such as scheelite (CaWO4).
Carbonates: minerals that are compounds of elements with the carbonate group - CO3 - such as magnesite (MgCO3), siderite (FeCO3) and calcite (CaCO3). This category also includes the extremely rare nitrates which are compounds of elements with the NO3 group.
Phosphates: minerals that are compounds of elements with the phosphate group - PO4 - such as monazite (CePO4) and xenotime (YPO4). Also in this category are the corresponding compounds with arsenic (the arsenates) and with vanadium (the vanadates).
Silicates: minerals that are compounds of elements with silicon and oxygen organized into the silicon-oxygen tetrahedron such as olivine ((Fe, Mg2)[SiO4]) and kyanite (Al2[SiO5]).
Physical Properties of Minerals.
Hardness is the resistance of a substance to abrasion or mechanical degradation. A scale of hardness, called Moh's Scale, has been established as a basis of camparison against which the hardness of a mineral may be compared. Developed by Friedrich von Moh, the scale consists of 10 common minerals each with an assigned hardness value from 1 (the softest) to 10 (the hardest) which are, from 1 to 10, talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and diamond.
A simpler and less expensive method of mineral identification is one that is based pricnipally on the recognition and analysis of a mineral's external (physical) properties. The following is a brief list of the principal physical criteria employed in the mineral identification process.
Mechanical Breakage is the response of a mineral specimen to a force sufficient to separate it intopieces. Minerals which break into fragments which possess one (or more) smooth, flat, planar surfaces are said to possess cleavage. Minerals which break into fragments which are rough and uneven are said to possess fracture.
Density is the ratio of a mineral's mass to its volume. Density is a very useful and reliable diagnostic feature, particularly if measured carefully with precise instruments since each mineral has a unique density.
Luster refers to the quality of light that is reflected from a mineral's surface. Minerals that shine like polished chrome or gold possess metallic luster, whereas those that do not possess nonmetallic luster. There are four additional subdivisions of nonmetallic luster, namely, vitreous (glassy), pearly, adamantine, greasy, pitchy, earthy and dull.
Color refers to the frequency and wavelength of light that is reflected from a mineral's surface. Color is perhaps the least useful of physical properties of a mineral, since the same mineral can possess many different colors (such as quartz).
Streak refers to the powdery residue which remains when a mineral is rubbed against an unglazed ceramic plate. The presence as well as the color of streak are used as a diagnostic criteria.
The other useful but lesser know physical properties are tenacity (the way a mineral responds when scratched by a needle), refractivity (the distortion of light transmitted through transparent minerals), radioactivity (the emission of subatomic particles and energy by unstable elements in the mineral), fluorescence (the emission of bright, colorful light by the mineral in response to the exposure of ultraviolet rays due to the presence of certain activator ions in the mineral), magnetism (the ability of a mineral to be attracted by a magnet) and acid reactivity (the effervescing or bubbling of a mineral when in contact with certain acids).
The purpose of this exercise is to acquaint you with the physical properties and characteristics of a few of the most common minerals present if the Earth's crust. Based on your reading of Minerals and Mineral Identification as well as a visit to specific common mineral webpages which can be accessed through the following link you are to answer the questions which appear below.
4. The hardness of the average person's fingernail is 3.5. Which of the minerals listed can be scratched by your fingernail?
5. Which of the minerals leaves a characteristic yellow streak?
6. Which of the minerals, if any, are elements?
7. Which of the minerals possesses a dark black streak, cubic crystals, three directions of cleavage at right angles to each other, a metallic luster and low hardness?
8. Which of the minerals possesses fracture, occurs in many colors, forms six-sided crystals, has a hardness of 7 and is the most common rock-forming mineral in the rocks of the Earth's crust?
9. Which of the minerals possesses fracture, occurs in many colors though typically red, forms beautiful, many-sided crystals, possesses a high hardness, is used both as a gemstone and as an abrasive, and is usually associated with metamorphic rocks like schist?
10. Which of the minerals is salmon to tan to pinkish white in color, possesses two directions of cleavage and a hardness of 6, and is the second most common rock-forming mineral in the rocks of the Earth's crust?