Photo of Orthoclase in Processed & Rough Form
Orthoclase is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Monoclinicly structured gems are made of potassium aluminum silicate, their full chemical compound being Kalsi3O8.
Orthoclase mineral is a silicate of potassium and aluminum, belonging to the Feldspar group. It has the same composition as microcline, but is stable at slightly higher temperatures.
It occurs as prismatic, sometimes flat-sided crystals, but in rocks it is usually anhedral. It may be perfectly transparent and yellow or almost colorless, but it is more often semi-opaque and white to grayish-white, yellowish white, pink, or reddish.
Clear, yellow orthoclase from the pegmatites of the Malagasy Republic is well-known and is used as a gemstone. Fine pink crystals are found in cavities in granite at Baveno (Novara) and in granophyre at Cussao al Monte (Varese), and white crystals at San Piero in Campo (Elba), all in Italy.
Classic twinned forms come from Carlsbad (Czechoslovakia), and well-formed red crystals from Kirkpatrick (Scotland), Flims (Grison, Switzerland), Marienburg (Germany), Striegau (Silesia), and various localities in the Soviet Republics. Blue crystals occur near Lake Baikal (Russia), gray at Villadreu (Spain). Remarkable crystals are found in lithoclases of crystalline-schistose rocks in the Zilleral (Austria), and Val di Vizze (Bolzano, Italy).
The somewhat rare variety of transparent orthoclase from Malagasy is called noble orthoclase. It is basically yellow in color and usable as a gem.
Its color varies from mid- to golden yellow, champagne-colored. It is perfectly transparent with vitreous luster. Gems are usually free of inclusions.
Noble orthoclase can be confused with several yellow stones.
What are the fundamental gemological properties?
Crystal System: Monoclinic Refractive Index: 1.518-1.526 Birefringence: 0.005-0.008 Specific Gravity: 2.56-2.58 Hardness: 6-6.5 Mohs Cleavage: Perfect {001}, good {010}
What determines color variations?
Pink: Iron oxidation state Yellow: Trace iron content Colorless: Pure material White: Common in massive forms Brown: Iron and manganese presence Color zoning frequent in larger crystals
What characterizes gem-quality material?
Transparency in crystalline specimens Uniform color distribution Minimal inclusions and fractures Well-developed crystal faces Absence of significant alterations Good polish potential
What are key formation indicators?
Forms in granitic and pegmatitic environments High-temperature polymorph of potassium feldspar Often shows simple twinning Common association with quartz and mica Temperature-dependent ordering structure
What are the cutting considerations?
Orientation critical due to cleavage Cabochon cuts most common Faceting possible with careful orientation Polish lines may show on cleavage faces Requires experienced cutting technique
What affects jewelry durability?
Cleavage limits use in impact-prone settings Better suited for pendants and earrings Requires protective mounting Temperature sensitive Avoid ultrasonic cleaning
What are diagnostic testing methods?
Optical testing confirms monoclinic structure Specific gravity distinguishes from similar feldspars X-ray diffraction confirms crystal system Chemical testing reveals potassium content UV typically inert to weak blue
The specific gravity [?] for Orthoclase is 2.56, its refractive index [?] is 1.51-1.54, and its double refraction [?] is 0.005.
History
Orthoclase got its name because it has two directions of cleavage, which are at right angles to one another.
Industrial Usages
Orthoclase mineral is important for industrial uses. The mixture orthoclase, kaolin, and quartz is easy to mold and fuses at a relatively low temperature, to a light, glassy, white, slightly porous mass known as porcelain. When very pure, it is used to make special porcelains (high-tension electric insulators, ceramic glazes, and dental products) and opalescent glass. For some years it has been used, finely ground and mixed with detergents, as a scouring powder.
The cut most commonly used with gem-quality noble orthoclase is the step cut. Its value is low, even compared with other secondary gems. But being somewhat rare, fine specimens are sought by collectors and connoisseurs. Noble orthoclase is neither imitated nor produced synthetically.
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