Orthoclase Feldspar | Gemopedia

While feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, as they often possess rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites, for example, "Black Pearl" granite countertops are actually composed mostly of feldspar. While orthoclase, a potassium-rich feldspar, is better known to collectors, there is one variety that many are familiar with--moonstone, prized for its shimmering adularescence.
Alternate
Names
Adularia, Moonstone, Sunstone
Colors
Colorless, White, Green, Orange, Yellow To Brown, Gray

Orthoclase Feldspar Classification

Common Name

Orthoclase Feldspar

Species

Orthoclase

Variety

Colors

Colorless, White, Green, Orange, Yellow To Brown, Gray

Alternate Names

Adularia, Moonstone, Sunstone

Gemstone Groups

Feldspar

Key Separations

refractive index, birefringence, optic character and possibly spectrum and fluorescence

Comments

Feldspar is a group of minerals of which orthoclase is one species. This information is illustrative of the yellow or orange-brown transparent faceted material depicted above, not necessarily other species and varieties within the group. Orthoclase feldspar can display a 4 rayed star.Adularia: colorless variety sometimes with adularescence.Moonstone: the semitransparent to opaque variety with white or blue adularescence. Sunstone: the semitransparent to opaque orange to brown variety that displays aventuresence.

Orthoclase Feldspar Optical Properties

Transparency

Transparent - Opaque

Refractive Index

1.518-1.526
Tolerance:(+0.010/-)0.010)

Birefringence

0.005-0.008

Optic Character

Biaxial

Optic Sign

Negative

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: transparent yellow: inert to weak reddish orange
LWUV: transparent yellow: inert to weak reddish orangesemitranslucent white: red

CCF Reaction

Pleochroism

Unobservable

Dispersion

Strength: weak fire Value: 0.12

Comments

Orthoclase Feldspar Chemistry & Crystallography

Chemical Name

potassium aluminum silicate

Chemical Formula

KAlSi3O8

Synthesis

Crystal System

Monoclinic

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline

Comments

Orthoclase Feldspar Characteristic Physical Properties

Hardness

6-6.5

Streak

White

Specific Gravity

2.55-2.61 Range:+/-0.03 Typical:2.58

Toughness

Poor

Inclusions

Orthoclase feldspar might display centipede like inclusions. Material with aventuresence or asterism might contain reflective inclusions such as fine needles or elongated hematite platelets.

Luster

Vitreous

Stability

Fracture

Uneven, Splintery

Cleavage

Perfect, in two directions

Comments

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