Minerals are: 1) naturally occuring 2) inorganic 3) solids with 4) definite chemical compositions and 5) an orderly arrangement of atoms. There are over 2000 different minerals. A few common "rock-forming minerals" make up most of the rocks in earth's crust, and most of these (over 90 percent) are silicates. The minerals are grouped based on their chemical composition. A few native minerals (gold, silver, copper, sulfur, diamond, ...) are made of a single element. Most minerals, however, are formed of compounds -- two or more chemically combined elements. Listed below are some important groups of rock-forming minerals. For a more extensive list of minerals and mineral groups you will need to use a rock & mineral guide or visit other online resources.

---

Common Rock-Forming Minerals

Silicates: minerals made of silica tetrahedrons (silicon + oxygen), alone or combined with other elements

Silicate Families :

• quartz - made completely of silica tetrahedrons; hardest common mineral (H=7 on the Moh's scale) and second most abundant in the earth's crust; color varies, but has a glassy luster ; found in granites, sand, and sandstone
• feldspar - most abundant family of minerals in earth’s crust, includes:
  • orthoclase (most common example of potassium feldspar) - cleaves in two directions that meet at 90 degrees; light color with a pearly luster; H=6; common in granites
  • plagioclase (sodium-calcite) feldspars - cleaves in two directions that meet at oblique angles; one surface often has striations; color varies from white to reddish brown; H=6; examples include albite & oligoclase
• mica - soft (H=2.5), flaky mineral; non-metallic luster; excellent cleavage in one direction that peels into thin elastic layers; includes muscovite (silvery white) and biotite (dark - brown/black)
• talc - softest mineral (H=1); color is white, gray or geenish; good cleavage in one direction; soapy feel; used in powders
• hornblende - most common mineral in the amphibole family of silicates; dark color; shiny luster; H=5-6; 2 good cleavages meet at oblique angles; form long slender crystals; common in many dark crystalline rocks
• augite - most common pyroxene mineral; dark color; H=5-6; two good cleavages at nearly 90 degrees; less luster than horneblende; forms stubby crystals; common in many dark crystalline rocks
• olivine - olive (yellowish) green; glassy luster; forms granular (grain-like) crystals; hard (about 6.5); found in dark crystalline rocks and some meteorites
• garnet - very hard (6.5 - 7.5); found in many crystalline rocks
• kaolin - aluminum silicate; forms from weathering of feldspars & other silicates; primary mineral in clay & shale

ferromagnesian silicates: dark colored minerals containing iron & magnesium; may belong to almost any of the silicate families; examples include hornblende, augite, olivine

---

Carbonates: made of carbonate group (carbon + oxygen) joined with various metal ions; found in limestone and marble

• calcite - "calcium carbonate"; clear or white when pure - may be any color however due to impurities; H=3; three perfect cleavages are oblique; forms rhombs; reacts to readily to acid; most common carbonate mineral
• dolomite - calcium magnesium carbonate; doesn’t react as easily to acid as calcite; found in some limestones & marbles
• copper carbonates include malachite (blue) and azurite (green); an iron carbonate is siderite (brown or yellow-brown)

---

Iron Oxides: oxygen combined with iron

• hematite - most common iron oxide; H=5-6; earthy red or silvery metallic luster; streak is always red-brown
• magnetite - dark iron oxide; H=5.5-6.5; attracted to a magnet; forms small grains or crystals in many rocks
• lodestone - magnetic form of magnetite; "natural magnet"

---

Iron Sulfides: sulfur combined with iron

• pyrite - most common sulfide mineral; pale brass to golden-yellow color; metallic luster; H=6-6.5; may form 6- or 12-sided crystals; common name is “fool’s gold”
• chalcopyrite - iron sulfide which also contains copper (Cu); brassy yellow, tarnishes to iridescent blues, greens & purples (sometimes called "peacock copper"); metallic H=3.5-4; contains less than a third copper, but because it is found in large quantities, it is the major source of copper

---

Other Minerals:

• gypsum - clear, white, yellow or reddish; vitreous luster; very good one direction cleavage; soft (H=2); forms of gyspsum used as base in plasters & cements, as well as fertilizers and filler for paper
• sulfur - yellow; H=1.5-2.5; indistinct cleavage; earthy luster; may smell like rotten egg; burns easily ( and has low melting point), but gives off a poisonous gas
• galena - ore containing 86% lead; excellent three directions of cleavage at 90 degrees; bluish lead gray color; metallic luster (when freshly broken); very dense; it is also the leading ore for silver, even though it only contains small amounts
• diamond - native mineral made of carbon; H=10 (hardest) on the Moh's scale
• halite - rock salt (NaCl); cleaves into cubes; can be identified by taste (not recommended)

---

Identification of Minerals

class set of minerals	mineral tests: acid, magnetism, hardness	measuring specific gravity

Minerals are identified by their physical & chemical properties.

Inspection:

• color - most obvious; often least useful
• luster - how light reflects off mineral, includes: metallic and nonmetallic (vitreous, pearly, glassy, dull, earthy...)
• crystal shape - intersection of the crystal faces; many crystals lacked time or room to develop adequately for visual inspection

Simple Tests:

• streak - color of minerals powder left on a streak plate
• cleavage - tendency to split along flat surfaces; number of cleaveage planes (one, two, three, or none) and angle of intersection between the planes (90 degrees or oblique) can help in identification of some mineral
• fracture - minerals that don't cleave; includes conchoidal (shell-like), splintery, irregular, ...
• hardness - measured using Moh's scale; 1 (talc - softest mineral) to 10 (diamond- hardest mineral)

Other Tests:

• specific gravity - ratio of the weight of a mineral to the weight of an equal volume of water (compares density of mineral to that of water)

specific gravity = (weight of mineral in air) / (weight in air - weight in water)

Example: (measurements of a mineral tested in class)

mass of mineral in air = 30 grams
mass of mineral in water = 18.5 grams
difference in mass (equals the mass of an equal volume of water) = 30.2 grams - 19.0 grams = 11.2 grams

specific gravity = 30.2 grams / 11.2 grams = 2.70

This measurement helped confirm the mineral's identity as calcite. According to the Dorling Kindersley handbook, Rocks and Minerals by Chris Pellant, the specific gravity of calcite is 2.71.

• acid test - calcite found in limestone and marble reacts readily to acid by fizzing (produces carbon dioxide gas)

Special Properties:

• magnetism - attracted to magnet (magnetite) or even acts as a magnet (lodestone)
• taste - halite (rock salt) can be identified using taste, but this is NOT a recommended test
• fluorescence - glows under ultraviolet light (flourite, ...)
• radioactivity - subatomic particles released are measured with a Geiger counter
• double refraction - splits light rays into two parts (Island spar -- a transparent variety of calcite)

return to top