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Natural Oligoclase

The feldspar group of minerals are important for the place they take in the formation of rocks. The feldspars which are closely allied in form and habit as well as mode of occurrence, are aluminous silicates of potassium, sodium, calcium and barium. They are usually divided into four sub groups, 1. orthoclase and microcline (potassium aluminum silicate); 2. albite (sodium aluminum silicate, 3. anorthite (calcium aluminum silicate), and 4. celsian (barium aluminum silicate).

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Oligoclase Profile

Color information: green, orange, red, colorless
Refractive Index: 1.53 - 1.54
Chemical Composition: [Na,Ca]AlSi3O8
Hardness: 6 - 6.5
Density: 2.62 - 2.65
Crystal Group: Triclinic
Ocurrence: Tvedestrand and Hitero on the south coast of Norway; near Lake Baikal Russia; Ontario, Canada; Tanzania; Kangayam, southern India; France; and in Maine, New Mexico, New York, North Carolina, Pennsylvania, Virginia and Oregon in the USA.
Sign of the Zodiac: N/A
Month of the year: N/A
The plagioclase series defines the feldspars in the albite - anorthite range with varying percentages of albite and anorthite. The series consists of albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Mineralogists have assigned various names to minerals falling in certain positions in the series as a function of their chemical composition and the percentages of albite and anorthite define the mineral. Oligoclase is 70 - 90% albite and 10 30% anorthite.

The most well known gem variety of oligoclase type is sunstone, a mineral containing inclusions of red and orange goethite or hematite or both which are disseminated in parallel orientation through the near colorless oligoclase matrix. They give the mineral a rich golden or reddish-brown color with specular reflections. The green variety appears to be more rare and is sometimes known as adventurine feldspar.

The SG of oligoclase varies from 2.62 to 2.65, the most common value being near 2.64. The mineral is for practical purposes inert under ultra-violet light but shows whitish glow when irradiated with X-rays.

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