Part C
Common Rock-forming Minerals
Only a small fraction of the over 5,500 known minerals are of major importance in most rock-forming minerals. There are several important rock-forming mineral groups, including silicates, carbonates, and others (like oxides, sulfides, sulfates, etc.), each with many different varieties of similar minerals. For instance, the mineral hornblende is a common, double-chain silicate mineral (inosilicate) and is an important member of the amphibole group. But only a few dozen rock-forming minerals compose most of the common rock types, and in this class, we will typically focus on the more common mineral representatives. As you progress deeper into geology, you will become familiar with many of the lesser known minerals.
Silicate minerals are composed of various arrangements of silicon (Si) and oxygen (O), and other elements. These are the most important rock-forming minerals and commonly (but not always) form at high temperatures from magmas or under various metamorphic environments within the Earth. Carbonate minerals are composed of carbon (C), oxygen (O), and other elements, and are also an important rock-forming mineral group. There are several other important groups of minerals, including sulfides, sulfates, oxides, etc.
Differences in mineral chemical composition and atomic structure give each mineral its physical properties, which then allow a mineral to be identified and classified. Color is an initial indication of the chemistry of a silicate mineral: light-colored minerals are richer in silica (SiO2) and also potassium (K), sodium (Na), and aluminum (Al), whereas the dark-colored minerals have less silica and are richer in iron (Fe), magnesium (Mg), and calcium (Ca). The shape of a mineral crystal, either by its crystal form or by how it breaks, can yield clues to its atomic structure. Let's look at some brief descriptions of the more common rock-forming minerals.
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Figure 4-15. Grand Canyon igneous and sedimentary minerals. At left, silvery muscovite, with its one cleavage plane, forms muscovite "books" in this pegmatite dike along the Hermit Trail. At right, large calcite crystals growing in a Redwall Limestone opening (~1 m wide) across from Nautiloid Canyon (mile 34.8). |
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Mineral Descriptions
Olivine / (Mg,Fe)2SiO4 / Granular or grainy appearance; pale to dark green glassy grains; looks like green sand or green sugar. H = 7. No obvious cleavage, but has conchoidal fracture. Forsterite is the Mg-rich olivine and Fayalite is the Fe-rich olivine. Common in mafic and ultramafic igneous rocks.
Amphibole / Ca2(Fe,Mg)4Al(Si7Al)O22(OH)2 / Elongated crystals that are typically black or greenish-black color. H = 5.5. Two cleavages form corners not at right angles (56º or 124º angles), so it looks diamond-like in cross section. A group of double-chained silicates minerals, including hornblende, tremolite, actinolite, etc. Common in many igneous and metamorphic rocks.
Pyroxene / (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 / Prism- or blocky-shaped crystals that are usually dark army green or black in color. Two cleavages intersect at nearly right angles (~90º). It may look octagonal in cross section. H = 5.5. A group of single-chained silicates minerals, including augite, enstatite, diopside, etc. Common in many mafic and ultramafic igneous, as well as metamorphic rocks.
Plagioclase feldspar / NaAlSi3O8 to CaAl2Si2O8 / Commonly tabular in shape with white to gray color. May have parallel "grooves" (striations) on the flat surfaces; Two cleavages intersects at nearly right angles (~90º). H = 6. A group of Na- to Ca-rich feldspars, including the Na-rich albite to the Ca-rich anorthite. Common in many igneous, metamorphic rocks, and clastic sedimentary rocks.
Alkali feldspar / NaAlSi3O8 to KAlSi3O8 / Tabular or blocky crystals that are white to pinkish to peach in color. May contain wavy colored lines of Na-felspar (exsolution lamellae). Two cleavages intersects at nearly right angles (~90º). H = 6.A group of Na- to K-rich feldspars, including the Na-rich albite to the K-rich sanidine, orthoclase, and microcline. Common in many felsic igneous rocks and metamorphic rocks.
Biotite / K(Mg,Fe)3(AlSi3O10)(OH)2 / Flaky and breaks into thin flexible sheets. Dark color, usually black to brownish-black. One perfect (basal) cleavage. Gray-brown streak. H = 3-2.5. A prominent member of the Mica group of sheet silicates. Common in many felsic to intermediate igneous rocks and metamorphic rocks.
Muscovite / KAl2(AlSi3O10)(OH)2 / Basal cleavage allows this mineral to break into thin, flexible sheets that individually may be transparent. Usually pale brown to silver in color. H = 2.5-2. A prominent member of the Mica group of sheet silicates. Common in many felsic igneous rocks and metamorphic rocks.
Quartz / SiO2 / Often irregular in shape, but can form hexagonal prism crystals. Depending on impurities, can have many colors. No cleavage, but has conchoidal fracture. H = 7. A very common framework silicate with many variations. Common in many felsic igneous rocks, metamorphic rocks, and sedimentary rocks.
Calcite / CaCO3 / Can have many forms, but cleavage causes it to break into rhombic shapes. Typically light-colored with many color varieties. Reacts vigorously with acid. H = 3. May display double refraction and fluorescence. A prominent member of the carbonate group of minerals. Common in chemical sedimentary rocks.
Gypsum / CaSO4 * 2H2O / Tabular shape or fibrous. Commonly clear or white colored. Two cleavages that intersect not at 90º (66º or 114º angles) form prismatic sheets. H = 2. An evaporite mineral common in sedimentary rocks.
Halite / NaCl / Forms cubic crystals. Typically light-colored with many color varieties. Three cleavages intersect at 90 to form cubes. H = 3. An evaporite mineral common in sedimentary rocks.
Pyrite / FeS2 / Metallic luster. Brassy yellow color that can tarnish brown. Often massive or forms cubes. Dark gray streak. H = 6.5-6. Specific gravity = 5.0. A sulfide mineral commonly known as "fools gold".
Hematite / Fe2O3 / Metallic to non-metallic luster. Metallic gray to earthy red color. Red to red-brown streak. H = 5.5. Specific gravity = 5.3-4.9. An iron oxide and common component in the cement of clastic sedimentary rocks.
Garnet / (Mg,Fe,Mn,Ca)3Al2Si3O12 / Commonly forms dodecahedral (12-sided) to cubic crystals. Can be red to green in color, but is typically reddish-brown. No cleavage. H = 7. A group of silicate minerals that include reddish almandine and pyrope. Common in metamorphic rocks and felsic igneous rocks.
Quiz Me! questions C21 through C26 refer to the mineral descriptions above. You can always view the question, then review the mineral descriptions above, and then return to the question to answer it.
Mineral Compositions
Let's answer a few questions about mineral compositions. Again, you can view the question, then review the mineral descriptions above, and then return to the question to answer it.
Mineral Identification
Most common minerals can be identified using a few important physical properties, like color, luster, hardness, cleavage, etc. It's actually pretty easy to identify most common minerals when you organize your observations of the important physical properties into a flow-chart-like decision tree. There are a few ways to proceed with identification.
Luster / Hardness / Streak
One approach to identifying a mineral is to first determine the mineral's luster (metallic or non-metallic). If the luster is metallic, then the next property to check the mineral's hardness (by scratching it with your fingernail, a penny, a steel nail, etc.). After this, you can look at the color of the mineral's streak. Lastly, any other characteristic properties (like magnetism, cleavage, crystal form, etc.) can be added to your observations. By following this process, the identity of the metallic mineral should reveal itself. If the mineral luster is non-metallic, then follow the same procedure as before. Determine the hardness and streak, although streak isn't as important (most nonmetallic minerals leave a white streak). Then look at cleavage and any other properties. The identity of the non-metallic mineral should also become clear.
> Click HERE to see a PDF chart for identifying minerals with metallic luster (from Boger et al., 1993).
> Click HERE to see a PDF chart for identifying minerals with non-metallic luster (from Boger et al., 1993).
Cleavage / Streak / Hardness
Another approach is to look to see whether a mineral possesses cleavage or not. If it does, you can next look at its streak. If necessary, you can describe the cleavage in more detail and then add any other helpful observations to identify the mineral. If the mineral lacks obvious cleavage, then streak, hardness, and luster can be observed to identify the mineral.
> Click HERE to see a PDF chart for identifying minerals with obvious cleavage (from Boger et al., 1993).
> Click HERE to see a PDF chart for identifying minerals that lack obvious cleavage (from Boger et al., 1993).
Quiz Me! questions C34 through C40 involve mineral identification. You'll recognize some minerals right away because they have distinctive physical properties, but others may require more in-depth study of the mineral descriptions, mineral images, or even the mineral identification charts. Make use of the hints, if necessary.