Earth

Rocks, Minerals, and Gemstones

Table 09-02 Evolution of Minerals

		Rocks are aggregates of minerals - usually several, but sometimes only one or two. Minerals are either free, uncombined native elements (such as gold, silver, and copper), or elemental compounds (such as silicates - metallic elements combine with the Si-O tetrahedral radical). Gemstones are minerals suitable for use in
Figure 09-06m Rocks, Minerals, and Gemstones [view large image]	Figure 09-06n Gemstones Worldwide [view large image]	jewelry after cutting and polishing (Figure 09-06m). They are rare, and therefore valuable, because their formation requires a highly unusual set of geological

		Since the Earth's crust composed mainly of Oxygen (46.6%) and Silicon (27.7%) for a total of 75%, the predominant compositions in minerals and thus in rocks are compounds such as quartz (SiO2), feldspars (XAl1-2Si3-2O8, where X can be either the elements Na, K, or Ca), and Mica (...Si3O10...) (see Figure 09-06o). There are three types of rocks according to the formation process (see Figure 09-06p, and Table 09-03). They are further sub-divided into different grain sizes and colors (light, medium, dark, not shown in the figure).
Figure 09-06o Minerals in Rocks [view large image]	Figure 09-06p Types of Rock

Type	Formation	Characteristic	Composition	Examples
Igneous	Solidified from molten magma either at the Earth's surface (extrusive) or underneath (intrusive).	The crystals can be very large (via slow cooling), and mostly have random distribution		Basalt, Granite
Metamorphic	Created when existing rock is chemically or physically modified by intense heat or pressure, e.g., in collision of crustal plates	Have either wavy foliation (layer) or more random arrangement		Gneiss, Schist
Sedimentary	Formed from erosion, transportation and subsequent deposition of pre-existing rocks or other kinds of sediments	May occur in layers, grains may be poorly held together		Shale, Sandstone

Table 09-03 The Three Types of Rock

1a,b. Magma (molten rock) inside the Earth's crust rises through cracks and cools slowly underground forming igneous rocks composed of minerals with fairly large crystal sizes, these are known as intrusive igneous rocks. When the magma erupts onto the surface, as through a volcano, it is termed lava, the rapid rate of cooling makes the extrusive igneous rocks to form with medium to very small mineral crystals. 2. Once on the surface, the forces of erosion and weathering produce smaller particles (sands), which accumulate and compactify by pressure from upper layers to become sedimentary layers (rocks). 3. When sedimentary and igneous rocks are subjected to intense heat and pressure such as in the collision of the crustal plates, they turn into metamorphic rocks. Some of these are uplifted back to the surface by tectonic action.

Figure 09-06q Rock Cycle [view large image]

Further increases in temperature and pressure melt the rock deep under the crust into magma to complete the rock cycle.

1. SolutionPrecipitation - Near surface water becomes weak acid solution (with CO2 dissolved in it) in which many minerals are soluble. Gems will form as the water evaporates (Figure 09-06r (1)). Hot water from hydrothermal deep under are sometimes highly acidic or alkaline, making an even better solvent for more types of minerals. The slower rates of cooling and/or evaporation allow for larger crystals to form. Many of

Figure 09-06r Gem Formation [view large image]

the world's highest quality specimens and metal ores have come form such environments (Figure 09-06r (2)). It may also appear as veins in the cracks (Figure 09-06r (3)).

Class	Composition	#	Location	Examples
Silicate	Metallic elements + Si-O	> 500	95% of all rocks	Quartz SiO2, Garnet Mn3Al2(SO4)3
Carbonate	Metallic elements + (CO3)-2	200	Marine and evaporitic settings	Calcite CaCO3, Dolomite CaMg(CO3)2
Sulfate	Metallic elements + (SO4)-2		Evaporitic settings, hydrothermal veins	Gypsum Ca(SO4)H2O, Barite Ba(SO4)
Halide	Metallic elements + halogen	100	Evaporitic settings	Halite NaCl, Fluorite CaF2
Oxide	Metallic elements + oxygen	> 250	Precipitates on Earth's surface	Ice H2O, Hematite Fe2O3
Sulfide	Metallic/Semi-metallic elements + sulfur	> 300	Metal ores	Pyrite FeS2, Chalcopyrite CuFeS2
Phosphate	Metallic elements + (AO4)-3, where A can be P, As or V		Phosphate in teeth and bones	Pyroxmangite Pb5(PO4)3Cl, Bayldonite (Cu,Zn)3Pb(AsO4)2(OH)2H2O
Element	Chemical elements and alloys		Mines	Gold Au, Sulfur S, Silicides Fe3Si
Organic	Carbon + hydrogen		Fossil fuels	Hydrocarbons CnH2n+2, CnH2n, CnHn

Table 09-04 Mineral Classification by Chemical Properties

Table 09-05 Gem Identification by Physical Properties

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