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.
