It has intermediate characteristics between albite and anorthite. The relationship between the optical properties, composition and structural state of these feldspars is also discussed and a new set of refractive index curves is presented; these have been drawn using data on the groundmass feldspars from hawaiites and mugearites. The various plagioclase feldspars are identified from each other by gradations in index of refraction and density in the absence of chemical analysis and/or optical measurements. Oligoclase by definition must contain 90-70% sodium to 10-30% calcium in the sodium/calcium position of the crystal structure. Properties of Oligoclase Properties/Propriétés optiques et autres: Macles possibles - Gemme, pierre fine - Fragile, cassant - Transparent - Translucide - Présent dans météorite - Refractive Index/Indice de réfraction: from 1,53 to 1,55 / de 1,53 à 1,55 Oligoclase Optical Properties Transparency Translucent - Opaque Refractive Index 1.539-1.547 Tolerance:(+0.004/-0.006) Birefringence 0.007-0.01 Optic Character Biaxial Optic Sign Negative Polariscope Reaction Oligoclase was named in 1826 by August Breithaupt from the Greek words όλίγος and κλάσις meaning little and to break because the mineral was thought to have a less perfect cleavage than Albite. Optical and misc. oligoclase (plagioclase) IM colourless 1.53-1.55 v.low +- -12 to 12 (Michel-Levy) 70-90, -ve or +ve low: up to white Albite twins along length, also Carlsbad + others In igneous rocks often with biotite and K-feldspar. Comments: Clear glassy crystalline oligoclase with milky quartz and muscovite. Location: Bakersville, Mitchell County, North Carolina, USA. The middle part of this crystal sparkles, and usually the color is darkest in the middle and becomes lighter toward the outer edges. Optical properties / Propriétés optiques. Also in low-med grade meta rocks. Oligoclase, a rock-forming mineral, is a member of the plagioclase feldspars. The optical effect called schiller and the color in Oregon Sunstone is due to copper. All plagioclase feldspars show a type of twinning that is named after albite. Oligoclase is about 90-70% Albite and 10-30% Anorthite. Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references. Optical Character: Biaxial/-Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references. The mineral was named by August Breithaupt in 1826 after the Greek words ‘oligos’ and ‘clasein’ which mean little and to break, respectively. Optical Properties of Oligoclase; Refractive Index: 1.538 to 1.550. Crystals in igneous rocks often rectangular laths. Often zoned: usually An-rich cores and Ab-rich rims. Birefringence: 0.010 .
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