(d). Composition of Rocks

A rock can be defined as a solid substance that occurs naturally because of the effects of three basic geological processes: magma solidification; sedimentation of weathered rock debris; and metamorphism. As a result of these processes, three main types of rock occur:

Igneous Rocks - produced by solidification of molten magma from the mantle. Magma that solidifies at the Earth's surface conceives extrusive or volcanic igneous rocks. When magma cools and solidifies beneath the surface of the Earth intrusive or plutonic igneous rocks are formed.

Sedimentary Rocks - formed by burial, compression, and chemical modification of deposited weathered rock debris or sediments at the Earth's surface.

Metamorphic Rocks - created when existing rock is chemically or physically modified by intense heat or pressure.

Most rocks are composed of minerals. Minerals are defined by geologists as naturally occurring inorganic solids that have a crystalline structure and a distinct chemical composition. Of course, the minerals found in the Earth's rocks are produced by a variety of different arrangements of chemical elements. A list of the eight most common elements making up the minerals found in the Earth's rocks is described in Table 10d-1.

 

Table 10d-1: Common elements found in the Earth's rocks.

Element	Chemical Symbol	Percent Weight in Earth's Crust
Oxygen	O	46.60
Silicon	Si	27.72
Aluminum	Al	8.13
Iron	Fe	5.00
Calcium	Ca	3.63
Sodium	Na	2.83
Potassium	K	2.59
Magnesium	Mg	2.09

 

Over 2000 minerals have been identified by earth scientists. Table 10d-2 describes some of the important minerals, their chemical composition, and classifies them in one of nine groups. The Elements Group includes over one hundred known minerals. Many of the minerals in this class are composed of only one element. Geologists sometimes subdivide this group into metal and nonmetal categories. Gold, silver, and copper are examples of metals. The elements sulfur and carbon produce the minerals sulfur, diamonds, and graphite which are nonmetallic.

Figure 10d-1: Silver.

 

Figure 10d-2: Copper.

 

Figure 10d-3: Graphite.

 

The Sulfide Group are an economically important class of minerals. Many of these minerals consist of metallic elements in chemical combination with the element sulfur. Most ores of important metals such as mercury (cinnabar - HgS), iron (pyrite - FeS₂ ), and lead (galena - PbS) are extracted from sulfides. Many of the sulfide minerals are recognized by their metallic luster.

Figure 10d-4: Pyrite.

 

Figure 10d-5: Galena.

 

The Halides are a group of minerals whose principle chemical constituents are fluorine, chlorine, iodine, and bromine. Many of them are very soluble in water. Halides also tend to have a highly ordered molecular structure and a high degree of symmetry. The most well-known mineral of this group is halite (NaCl) or rock salt.

Figure 10d-6: Halite or rock salt.

 

The Oxides are a group of minerals that are compounds of one or more metallic elements combined with oxygen, water, or hydroxyl (OH). The minerals in this mineral group show the greatest variations of physical properties. Some are hard, others soft. Some have a metallic luster, some are clear and transparent. Some representative oxide minerals include corundum, cuprite, and hematite.

Figure 10d-7: Corundum

 

Figure 10d-8: Hematite

 

The Carbonates Group consists of minerals which contain one or more metallic elements chemically associated with the compound CO₃ . Most carbonates are lightly colored and transparent when relatively pure. All carbonates are soft and brittle. Carbonates also effervesce when exposed to warm hydrochloric acid. Most geologists considered the Nitrates and Borates as subcategories of the carbonates. Some common carbonate minerals include calcite, dolomite, and malachite.

Figure 10d-9: Calcite.

 

Figure 10d-10: Dolomite.

 

The Sulfates are a mineral group that contain one or more metallic element in combination with the sulfate compound SO₄ . All sulfates are transparent to translucent and soft. Most are heavy and some are soluble in water. Rarer sulfates exist containing substitutions for the sulfate compound. For example, in the chromates SO₄ is replaced by the compound CrO₄ . Two common sulfates are anhydrite and gypsum.

Figure 10d-11: Gypsum.

 

The Phosphates are a group of minerals of one or more metallic elements chemically associated with the phosphate compound PO₄ . The phosphates are often classified together with the arsenate, vanadate, tungstate, and molybdate minerals. One common phosphate mineral is apatite. Most phosphates are heavy but soft. They are usually brittle and occur in small crystals or compact aggregates.

The Silicates are by far the largest group of minerals. Chemically, these minerals contain varying amounts of silicon and oxygen. It is easy to distinguish silicate minerals from other groups, but difficult to identify individual minerals within this group. None are completely opaque. Most are light in weight. The construction component of all silicates is the tetrahedron. A tetrahedon is a chemical structure where a silicon atom is joined by four oxygen atoms (SiO₄). Some representative minerals include albite, augite, beryl, biotite, hornblende, microcline, muscovite, olivine, othoclase, and quartz.

Figure 10d-12: Albite.

 

Figure 10d-12: Biotite.

 

Figure 10d-13: Hornblende.

 

Figure 10d-14: Olivine.

 

Figure 10d-15: Orthoclase.

 

Figure 10d-16: Quartz.

 

The Organic minerals are a rare group of minerals chemically containing hydrocarbons. Most geologists do not classify these substances as true minerals. Note that our original definition of a mineral excludes organic substances. However, some organic substances that are found naturally on the Earth that exist as crystals that resemble and act like true minerals. These substances are called organic minerals. Amber is a good example of an organic mineral.

 

Table 10d-2: Classification of some of the important minerals found in rocks.

Group		Typical Minerals (and information link)	Chemistry
Elements		Gold	Au
		Silver	Ag
		Copper	Cu
		Carbon (Diamond and Graphite)	C
		Sulfur	S
Sulfides		Cinnabar	HgS
		Galena	PbS
		Pyrite	FeS2
Halides		Fluorite	CaF2
		Halite	NaCl
Oxides		Corundum	Al2O3
		Cuprite	Cu2O
		Hematite	Fe2O3
Carbonates (Nitrates and Borates)		Calcite	CaCO3
		Dolomite	CaMg(CO3)2
		Malachite	Cu2(CO3)(OH)2
Sulfates		Anhydrite	CaSO4
		Gypsum	CaSO4 -2(H2O)
Phosphates (Arsenates, Vanadates, Tungstates, and Molybdates)		Apatite	Ca5(F,Cl,OH)(PO4 )
Silicates		Albite	NaAlSi3O8
		Augite	(Ca, Na)(Mg, Fe, Al)(Al, Si)2O6
		Beryl	Be3Al2(SiO3)6
		Biotite	K (FE, Mg)3AlSi3O10(F, OH)2
		Hornblende	Ca2(Mg, Fe, Al)5(Al, Si)8O22(OH)2
		Microcline	KAlSi3O8
		Muscovite	KAl2(AlSi3O10)(F, OH)2
		Olivine	(Mg, Fe)2SiO4
		Orthoclase	KAlSi3O8
		Quartz	SiO2
Organics		Amber	C10H16O

 

 

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