An ore is defined as a mineral or rock from which some valuable constituent, usually a metal, can be profitably extracted. Thus hematite and magnetite are ores of iron; galena is an ore of lead; zinc blende is an ore of zinc. With the ore-mineral is more or less material of no value, called gangue; … Read more
Rocks that have been formed from other rocks, sedimentary and igneous, by internal changes caused by pressure, heat, and the chemical action of the solutions and hot gases, are called metamorphic rocks and here is a list: Crystalline Limestone Crystalline Limestone is a rock that results from the crystallization of the calcite in sedimentary limestone. … Read more
Porphyry. — This name refers to the structure, which is formed of larger crystals set in a fine-grained or glassy ground-mass. The porphyritic structure may be plainly visible, the larger crystals being easily seen (phenocrysts), or it may require the microscope to bring it out. Porphyritic structure may appear in any igneous rock, but the … Read more
We can list volcanic rocks that are formed at or near the surface by comparatively quick cooling of lava, cannot be classified without the aid of the microscope and chemical analysis, except roughly as light-colored and dark lavas, conveniently called rhyolite, and basalt or trap. Rhyolite Rhyolite is made of light-colored acid lava flows, often called … Read more
It is strongly recommended that these descriptions and examples of plutonic rocks be read with the specimens in hand, obtainable, as noted before, from the Geological Survey. Granite Granite is a coarse or medium- grained rock, composed of quartz, an acid feldspar or two of them (orthoclase, microcline, albite), and generally mica (biotite, or muscovite, or … Read more
Volcanoes A volcano is the most striking evidence of a hot interior. There are about 250 active volcanoes at present, and about as many more that have been active in historic times. The cone shape of a volcano is due to its gradual building up by the ejection of material from the center hole, the … Read more
The Earth As a preparation for the systematic study of rocks, it will be useful to look at the earth as a whole, inquiring into its origin, the changes that it has undergone and is still undergoing, and the nature and causes of the great variety of surface features. Origin of the Earth From comparison … Read more
The group of sulphates correspond to sulphuric acid, H2SO4. In the list of sulphate minerals, there are metals in place of the hydrogen of the acid; they are to be distinguished from sulphides, which contain no oxygen, but are made up of sulphur and metals. Barite or Barytes Barite or Barytes; BaSO4. — Also called … Read more
The Phosphate Mineral Group includes a number of the rarer minerals and apatite. Pitchblende or Uraninite is of Color, pitchy black, greenish, or brownish black; powder, brownish black, grayish, greenish; luster, dull and pitchy, sometimes slightly metallic. H = 5.5; G = 9 to 9.7; no cleavage; contains uranium, radium, etc. Found in pegmatite granite dikes, … Read more
Mica type minerals make a group of silicates of alumina, with potash, iron, magnesia; characterized by easy cleavage into thin, flexible, elastic sheets. Mica gives off water when strongly heated. Muscovite Muscovite, or white mica, (K,H) AlSiO4, is colorless, sometimes gray, brown, ruby, etc.; powder, white; luster, glassy or pearly; H = 2 to 2.5; G … Read more