Geology by Credo Reference: Rocks and Minerals

Sediments are loose, unconsolidated rock particles and ions (i.e., charged atoms) dissolved in aqueous solutions. Sedimentary rocks, which constitute about 75% of all rocks exposed at the surface of the Earth, form when the particles are cemented together or when crystals precipitate from the solutions.

The so-called crust of the Earth is about 35 km thick under the continents but averages only some 7 km beneath the oceans. It is formed mainly of rocks of relatively low density.

Rock altered in structure, composition, and texture by pressure or heat after its original formation.

The volcanic equivalent of diorite, andesite is fine grained or porphyritic, and primarily consists of the plagioclase feldspar minerals andesine and oligoclase, plus one or more of the dark, ferromagnesian minerals, such as pyroxene or biotite.

An intrusive igneous rock, anorthosite is composed of at least 90 per cent calcium-rich plagioclase feldspar – principally labradorite and bytownite.

Fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color.

A soft, fine-grained, easily pulverized, white to grayish variety of limestone, chalk is composed of the calcite shells of minute marine organisms.

Colour When fresh it is black, dark grey or green; may be mottled black and white.

Colour Speckled black and white in hand specimen; occasionally shades of dark green or pink. The dark minerals are more noticeable than in gabbro.

Colour: White, cream or grey, but often weathers brown or pinkish. Texture: Coarse, medium or fine; compact, sometimes earthy.

Colour Grey, dark grey, black; may have a bluish or greenish tone. Colour index 30 to 90; with a decrease of coloured minerals gabbro grades into anorthosite, and with an increase it grades into pyroxenite and peridotite.

Coarse-grained intrusive igneous rock, typically consisting of the minerals quartz, feldspar, and biotite mica. It may be pink or grey, depending on the composition of the feldspar. Granites are chiefly used as building materials.

The major source of diamonds, kimberlite is a form of peridotite. It is mica-rich, often with well-formed crystals of brown phlogopite mica.

Molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure.

Metamorphic rock composed wholly or in large part of calcite or dolomite crystals, the crystalline texture being the result of metamorphism of limestone by heat and pressure.

Pegmatite is the name given to very coarse-grained, igneous rocks.

One of the most economically important rocks, peridotite is the ultimate source of all chromium ore, and its variety kimberlite is the source of all naturally occurring diamonds.

A very porous, froth-like volcanic glass, pumice is created when gas-saturated liquid magma erupts like a fizzy drink released from a shaken bottle, and cools so rapidly that the resulting foam solidifies into a glass full of gas bubbles.

Color Usually light coloured; white, grey, greenish, reddish or brownish. The colour may be even, or in bands of differing shades.

Syenites may be visually similar to granite, and are often confused with it, but they can be distinguished from it by the absence or scarcity of quartz.

A naturally occurring, silica-rich (65%–80% SiO2) glass resembling obsidian and sometimes shale, and is normally jet black to olive green.

TUFF IS A BROAD TERM used to include any relatively soft, porous rock made of lithified pyroclastic minerals (ash and other sediments ejected from volcanic vents).

Chemical compound of hydrogen and oxygen elements - H2O. It can exist as a solid (ice), liquid (water), or gas (water vapour).

Rock material occurring in the form of loose, rounded or angular grains, varying in size from .06 mm to 2 mm in diameter, the particles being smaller than those of gravel and larger than those of silt or clay.

Inorganic substance occurring in nature, having a characteristic and homogeneous chemical composition, definite physical properties, and, usually, a definite crystalline form.

Constituent of the Earth's crust composed of minerals or materials of organic origin that have consolidated into hard masses.

Antimony exists in two allotropic forms (see allotropy); the more common is silvery blue-white and has a rhombohedral crystalline structure.

Barium is a soft, silver-white, chemically active, poisonous metal with a face-centered cubic crystalline structure.

Beryllium is a strong, extremely light, high-melting, silver-gray metal with a close-packed hexagonal crystalline structure

Cobalt is one of the mineral elements known to be essential for health, but only required in minute amounts - a trace element.

Red-brown, very malleable and ductile, metallic element, atomic number 29, relative atomic mass 63.546.

Gold is very ductile and is the most malleable metal; it can be beaten into extremely thin sheets of gold leaf.

Lead (Pb) has been used since ancient times. It is commonly used in modern petroleum engineering, atomic reactors, and many organic (synthetic) compounds.

Lithium is a soft, silver-white metal.

Manganese is a pinkish-gray, chemically active metal.

General term for a large group of minerals, hydrous silicates of aluminum and potassium, often containing magnesium, ferrous iron, ferric iron, sodium, and lithium and more rarely containing barium, chromium, and fluorine.

Molybdenum is a hard, malleable, ductile, high-melting, silver-white metal with a body-centered cubic crystalline structure.

Nickel is a hard, malleable, ductile, lustrous, silver-white metal with a face-centered cubic crystalline structure.

Pure platinum is a malleable, ductile, lustrous, silver-white metal with a face-centered cubic crystalline structure.

The most abundant mineral, quartz is found in myriad colours, shapes, and varieties.

Pure silver is nearly white, lustrous, soft, very ductile, malleable, and an excellent conductor of heat and electricity.

Brittle, pale-yellow, non-metallic element, atomic number 16, relative atomic mass 32.064.

Tantalum is a rare, hard, blue-gray metal with a body-centered cubic crystalline structure. I

Tungsten is a very hard, silver-white to steel-gray metal with a body-centered cubic crystalline structure.

Hard, lustrous, silver-white, malleable and ductile, radioactive, metallic element of the actinide series, atomic number 92, relative atomic mass 238.029.

Vanadium is a soft, ductile, silver-grey metal.

Zinc is a lustrous bluish-white metal.