Oligoclase | Gemopedia

Oligoclase is a member of the plagioclase feldspar group. Two oligoclase varieties, sunstone and moonstone, are prized for their phenomenal optical properties.
Colors
Yellow, Brown To Orange, Brownish Red, Colorless, White To Light Green, Gray

Oligoclase Classification

Common Name

Oligoclase

Species

Oligoclase

Variety

Colors

Yellow, Brown To Orange, Brownish Red, Colorless, White To Light Green, Gray

Alternate Names

Gemstone Groups

Feldspar

Key Separations

appearance, RI, birefringence and optic character.

Comments

If adventurescence is present then the stone is sunstone.

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

Doubly Refractive (DR)

Fluorescence

SWUV: Inert
LWUV: Inert

CCF Reaction

Pleochroism

Unobservable

Dispersion

Comments

Oligoclase Chemistry & Crystallography

Chemical Name

calcium or sodium silicate

Chemical Formula

NaAlSi3O8 or CaAl2Si2O8

Synthesis

Crystal System

Triclinic

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline to Aggregate

Comments

Oligoclase Characteristic Physical Properties

Hardness

6-6.5

Streak

Specific Gravity

2.62-2.67 Typical:2.65

Toughness

Poor

Inclusions

Oligoclase feldspar has parallel closely spaced twinning planes. Transparent material might show eye-visible iridescent platelets.

Luster

Vitreous

Stability

Good

Fracture

Uneven, Splintery

Cleavage

Perfect, in two directions

Comments

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