Orthoclase (variety of Feldspar)
Current inventory:  0 gems
 

  
Orthoclase is named from the Greek words orthos meaning right and kalo meaning I cleave because its cleavages are at right angles.

First cited in 1801; IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

KAlSi₃O₈

Potassium Aluminum Silicate

Molecular Weight:

278.33 gm

Composition:

Potassium

14.05 %

K

16.92 %

K₂O

Aluminum

9.69 %

Al

18.32 %

Al₂O₃

Silicon

30.27 %

Si

64.76 %

SiO₂

Oxygen

45.99 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/J.06-40

Nickel-Strunz 10th Ed. ID:

9.FA.30

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H₂O
A : Tektosilicates without additional non-tetrahedral anions

Related to:

Feldspar Group. Celsian-Orthoclase Series. Polymorph of Microcline and Sanidine.

Members of Group:

Feldspar Group: Albite, Amazonite, Andesine, Anorthite, Anorthoclase, Banalsite, Buddingtonite, Bytownite, Celsian, Dmisteinbergite, Hyalophane, Labradorite, Microcline, Oligoclase, Orthoclase, Paracelsian, Reedmergnerite, Sanidine, Slawsonite, Stronalsite, Svyatoslavite 

Varieties:

Aglaurite, Barian Orthoclase, Delawarite

Synonyms:

Argillyite, Common Feldspar, Cottaite, Leelite, Muldan, Murchisonite

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Crystals commonly short prismatic along [100] or [001], tabular on [010], to 20 cm. Cleavable, granular, massive.

Twinning:

Common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws. 

Physical Properties

Cleavage:

Perfect on {001} and {010}; partings on {100}, {110} and {201}

Fracture:

Conchoidal to irregular/uneven

Tenacity:

Brittle

Moh's Hardness:

6.0

Density:

2.55 - 2.63 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable; GRapi = 200.97 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Colorless, white, gray, pale yellow, flesh-red, green; colorless in thin section; may exhibit opalescence or schiller iridescence

Transparency:

Transparent to translucent

Luster:

Vitreous, pearly on cleavages

Refractive Index:

1.518 - 1.525  Biaxial ( - )

Birefringence:

0.004 - 0.005

Dispersion:

Distinct, r > v

Pleochroism:

None

Occurances

Geological Setting:

The common feldspar of granites, granite pegmatites, and syenites. In cavities in basalts; in high-grade metamorphic rocks and as a result of potassic hydrothermal alteration; also authigenic and detrital.

Common Associations:

Albite, Muscovite, Biotite, Hornblende, Schorl, Beryl

Common Impurities:

Na, Fe, Ba, Rb, Ca

Type Locality:

Unknown

Year Discovered:

First cited in 1801

View mineral photos:

Orthoclase Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Orthoclase is a member of the Potassium Feldspars of the Feldspar Group of minerals that also includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Orthoclase was named orthose in 1801 by Rene Just Haüy from the Greek word orthos meaning right in allusion to the cleavage angle. The name was changed to orthoklase in 1823 by Johann Friedrich August Breithaupt. He added klase from the Greek word kalo meaning I cleave because its cleavages are at right angles. The sense of Haüy's naming of Orthoclase was that the mineral was a Feldspar, but he did not specify a type locality or a chemical analysis.

Orthoclase is a component of many rocks, especially alkalic and plutonic acid rocks, also granites, pegmatites and syenites. Feldspars are the most common minerals on the Earth's surface. Gem quality Orthoclase crystals are found at several locations around the world but Madagascar produces the largest cuttable Orthoclase known. Crystals from Itrongahy, Madagascar are very fine, transparent yellow and very large, up to 250 carats. Yellow and colorless catseye gems are known from the gem gravels of Burma (Myanmar) and Sri Lanka.

Orthoclase distribution: Widespread. Fine examples from St. Gotthard, Ticino, and at Val Giuv, Tavetsch, Graubünden, Switzerland. In the Zillertal, Tirol, Austria. From Baveno, Piedmont, in the Pfitschtal, Trentino-Alto Adige, and at San Piero in Campo, Elba, Italy. At Epprechtstein, Bavaria, Carlsbad, Bohemia, and Manebach, Thuringia, Germany. From Cornwall, England. In Russia, from the Mursinka-Alabashka area, near Yekaterinburg (Sverdlovsk), Ural Mountains. In the USA, from Maine, at Paris and Buckfield, Oxford County; at Cornog, Chester County, and Blue Hill and Lieperville, Delaware County, Pennsylvania. In California, from the Pala and Mesa Grande districts, San Diego County; in Colorado, on Mt. Antero, Chaffee County; at Crystal Pass, Goodsprings, Clark County, Nevada. From Guanajuato, Mexico. At Tanokamiyama, Shiga Prefecture, Japan. Gem crystals from Ampandrandava, Fianarantsoa, and Itrongay, near Betroka, Madagascar.
 

  
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