Rocks - Characteristics & Minerals

ROCKS - CHARACTERISTICS & MINERALS Rock is a naturally occurring solid aggregate of minerals and/or mineraloids. The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, and petrology is an essential component of geology. Metamorphic Rock, rock which has had its original composition and texture altered by heat and pressure deep within the Earth's crust. Metamorphism that is a result of both heat and pressure is referred to as dynamothermal, or regional; metamorphism produced by the heat of an intrusion of igneous rock is termed thermal, or contact. Sedimentary Rock, in geology, rock composed of geologically reworked materials, formed by the accumulation and consolidation of mineral and particulate matter deposited by the action of water or, less frequently, wind or glacial ice. Most sedimentary rocks are characterized by parallel or discordant bedding that reflects variations in either the rate of deposition of the material or the nature of the matter that is deposited. Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies. Igneous rocks are one of the three main types of rocks; the other types are sedimentary rocks and metamorphic rocks. Of the three types of rocks, only igneous rocks are formed from melted material. The two most common types of igneous rocks are granite and basalt. Granite is light colored and is composed of large crystals of the minerals quartz, feldspar, and mica. Basalt is dark and contains minute crystals of the minerals olivine, pyroxene, and feldspar.

These are the physical properties most useful for mineral identification:

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Color - Color in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light. The color of light is determined by its wavelength.

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Luster - is a description of the way light interacts with the surface of a crystal

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Transparency (or diaphaneity) - also known technically as diaphaneity, is a function of the way light interacts with the surface of a substance.

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Twinning - Twins form as a result of an error during crystallization. Instead of a "normal" single crystal, twins will often appear doubled where two crystals appear to be growing out of or into each other, like Siamese twins

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Cleavage - is a smooth break producing what appears to be a flat crystal face. Here are a few rules about cleavage.

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Fracture - a description of the way a mineral tends to break. It is different from cleavage and parting which are generally clean flat breaks along specific directions.

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Hardness - one measure of the strength of the structure of the mineral relative to the strength of its chemical bonds

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Specific Gravity - is a measure of the density of a mineral.

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Streak - closely related to color, but is a different property because the color of the mineral may be different than the color of the streak.

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Associated Minerals -

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Notable Localities Below is a list of some coloring elements and the color they produce in at least one mineralCobalt, Co, produces the violet-red color in erythrite, (cobalt arsenic sulfide).

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Chromium, Cr, produces the color orange-red color of crocoite, (lead chromate).

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Copper, Cu, produces the azure blue color of azurite, (copper carbonate hydroxide).

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Iron, Fe, produces the red color of limonite, (hydrated iron oxide hydroxide).

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Manganese, Mn, produces the pink color of rhodochrosite, (manganese carbonate).

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Nickel, Ni, produces the green color of annabergite, (hydrated nickel arsenate).

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Uranium, U, produces the yellow color of zippeite, (hydrated potassium uranyl sulfate hydroxide).

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Vanadium, V, produces the red-orange color of vanadinite, (lead vanadate chloride).