Chrysolite Group - Chrysolite or Olivine, Peridot
Composition. Orthosilicate of magnesium, with varying amounts of ferrous iron, (MgFe)2SiO4he ratio between the magnesium and iron varies widely.
Crystallization. Orthorhombic. Crystals usually a combination of prism, macro- and brachypinacoids and domes, pyramid and base. Often flattened parallel to either the macro-or brachy pinacoid.
Structure. Usually in embedded grains or in granular masses. Physical Properties. H. = 6.5-7. G. = 3.27-3.37. Vitreous luster. Olive to grayish green, brown. Transparent to translucent.
Tests. Infusible. Rather slowly soluble in hydrochloric acid and yields gelatinous silica upon evaporation. After operation to dryness, take up residue in water with nitric acid, filter off silica, add ammonia in excess to precipitate ferric hydroxide, filter, add ammonium oxalate to prove absence of calcium, add sodium phosphate and abstain precipate of ammonium-magnesium phosphate (test for magnesium). Distinguished usually by its glassy luster, green color and granular structure.
Occurrence. A rather common rock-making mineral, varying from an accessory character to that of a main constituent of the rock. It is found principally in the dark colored ferro-magnesium igneous rock such as gabbros, peridodite and basalt. A rock, known as united, is made up almost wholly of chrysolite. Found also at times as glassy grains in meteorites. Occasionally in crystalline dolomitic limestones. Associated often with pyroxene, the plagioclase feldspars, magnetite, corundum, chromite, serpentine, etc. The transparent green variety, known as periodic, and used as a gem material, was found in ancient times in the East, the Exact, the exact locality for the stones not being known. At present periodic is found in Upper Egypt, near the Red Sea, and in rounded grains associated with Crystals of chrysolite are found in the lavas of Vesuvius. Larger crystals, altered to serpentine, come from Snarum, Norway. Chrysolite occurs in granular masses in the volcanic bombs in the Eiffel. Dunite rocks are found at Dun Mountain, New Zealand, and with the corundum deposit of North Carolina.
Alteration. Very readily altered to serpentine; magnesium carbonate, iron ore, etc., may for at the same time.
Name. Chrysolite means golden stone. Olivine derives its name form the usual olive-green color of the mineral, and is the term usually given to the spices when speaking of it as a rock making mineral. Peridot is an old name for the species.
Use. As the clear green variedly, known usually as periodic, is has some use as a gem.
Other members of the Chrysolite Group which are rarer in occurrence are montocelite, CaMgSio4; fosterite, Mg2SiO4; and fajalite, Fe2 SiO4. Ordinarily chrysolite is intermediate in composition between the last two. Another member which has been found in the zinc deposited at Franklin furnace, New Jersey, is tephroite, Mn2SiO4.
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