Opal Composition, Crystallization & Structure
Composition. Silicon dioxide, like quartz, like quartz, with a varying amount of water, SiO2nH2O. A mineral – gel.
Crystallization. Amorphous.
Structure. Massive; often botryoidally, stalactitic, etc.
Physical Properties. H. = 5.5-6.5. g. = 1.9-2.3. Vitreous luster; often somewhat resinous. Colorless, white, pale shades of yellow, red, brown, green, gray and blue. With darker colors, which are due to various impurities. Often has a milky or “opalescent” effect and sometimes shows a fine play of colors. Transparent to opaque.
Test. infusible. Insoluble. Reacts like quartz. Gives a little water upon intense ignition in C.T.
Varieties. Precious Opal. White, milky blue, yellow. Sometimes dark, as in so-called black opal. Translucent, with an internal play of colors. This phenomenon is said to be due to thin curved laminate which refract the light differently from the mass of the material, and so serve to break it up into the various prismatic colors. Fire opal is a variety with intense orange to red reflections.
Common Opal. Milk-white, yellow, green, red, etc., without internal reflections.
Hyalite. Clear and colorless opal with a globular or botryoidally structure.
Geyserites. Opal deposited by hot springs and geysers. Found about the geysers in the Yellowstone Park.
Wool Opal. Fossil wood with opal as the petrifying material.
Tripolite, or Infusorial earth. Fine-friend deposits, resembling child in appearance. Formed by the accumulation of the siliceous shells of small sea organisms.
Occurrence. Opal is found lining and filling cavities in igneous and sedimentary rocks, where it has evidently been deposited through the agency of hot waters. Deposited from hot springs and occurs in sedimentary beds due to the accumulation of siliceous skeletons of minute sea animals. In its ordinary variety it is of widespread occurrence. Precious opals are found at Czernowitza, Hungary; in Queretaron and other states in Mexico; in Honduras; and from various localities in Australia, the chief district being White Cliffs, New South Wales. Recently black opal has been found in Nevada and Idaho.
Use. As a gem. The stones are usually cut in round shapes, en cabochon. Stones of large size and exceptional quality are very highly prized.
EXPERIMENTAL FLOTATION Mining Glossary C Previous Environmental Disasters Refining Sulfide Metal Concentrates Porphyry Deposits Channel in Vein Gallery Seismic prospecting Methods Flotation agents: Collectors, frothers, or modifiers Breaking Mechanics: Impact, sledging, and pressure crushing in Roll Crushers Transportation use in Prospecting Geological Risks in Junior Mining Company Operating characteristic of various Types of Rock Crushers Mineral Deposits Gyratory reduction crushers- types and characteristics Straight-Face Concaves
