Orthoclase

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Madagascar
12.18 carats
© Rarestone.com

Orthoclase is transparent to opaque, often colourless or champagne colour Potassium Feldspar.

The name derived from Greek "straight fracture" and refers to the two cleavages present in this mineral that meet virtually at right angles to each other.

Orthoclase Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
 
 
 
 
 
 

Orthoclase Gemstones by Size

This table shows distribution of Orthoclase gemstone sizes that are listed on this site. This can give a good indication as to the general availability of this gemstone in different sizes.
Contributed photos
Lightest:0.87 cts
Heaviest:37.22 cts
Average:11.35 cts
Total photos:71
Do you have a larger Orthoclase? Why not upload a photo?
0.87ct to 4.51ct4.51ct to 8.14ct8.14ct to 11.78ct11.78ct to 15.41ct15.41ct to 19.05ct19.05ct to 22.68ct22.68ct to 26.32ct26.32ct to 29.95ct29.95ct to 33.59ct33.59ct to 37.22ct
General Information
A variety or type of:Feldspar
Varieties/Types:
Adularia - A gemmy colourless variety of Orthoclase.
Moonstone - A variety of Orthoclase that exhibits a strong blue adularescence.
Sanidine - A triclinic Potassium Feldspar found in lavas.
Chemical Formula
KAlSi
 
3
O
 
8
Michael O’Donoghue, Gems, Sixth Edition (2006)
More from other references
Photos of natural/un-cut material from mindat.org
Physical Properties of Orthoclase
Mohs Hardness6
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Specific Gravity2.54 to 2.61
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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TenacityBrittle
Michael O’Donoghue, Gems, Sixth Edition (2006)
Cleavage QualityPerfect
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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FractureUneven
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Optical Properties of Orthoclase
Refractive Index1.518 to 1.536
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Optical CharacterBiaxial/-
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Birefringence0.005 to 0.008
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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PleochroismWeak
Michael O’Donoghue, Gems, Sixth Edition (2006)
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DispersionLow: 0.012
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Colour
Colour (General)Colourless, yellow
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Causes of ColourYellow, Fe3+ in tetrahedral coordination. Pink to red, microscopic hematite and/or lepidocrocite inclusions
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
LustreVitreous,Pearly
Michael O’Donoghue, Gems, Sixth Edition (2006) May be pearly on cleavage surfacesMore from other references
Fluorescence & other light emissions
Fluorescence (General)Yellow stones show weak orangy-red
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Fluorescence (Short Wave UV)Green orthoclase: moderate green
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Fluorescence (Long-Wave UV)Weak blue or cream. Occasionally weak-to-strong reddish orange.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Crystallography of Orthoclase
Crystal SystemMonoclinic
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
HabitTabular
Arthur Thomas, Gemstones (2009)
Geological Environment
Where found:Orthoclase is a component of many rocks and is typically found in acidic and alkaline igneous rocks particularly in granite pegmatites and syenites.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Inclusions in Orthoclase
-empty-
Further Information
Mineral information:Orthoclase information at mindat.org
Significant Gem Localities
Austria
 
  • Salzburg
    • Hohe Tauern
      • Habach valley
        • Nasenkopf
[var: Adularia]
Burma (Myanmar)
 
  • Mandalay Division
    • Pyin-Oo-Lwin District
      • Mogok Township
        • Bernardmyo
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Chaung-gyi
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Kin (Guard Post)
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Kyatpyin Central
Ted Themelis (2008) Gems & mines of Mogok
Ted Themelis (2008) Gems & mines of Mogok
        • Kyatpyin North
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Kyauk-Pyat-That
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Marble Ark
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Mogok Valley
          • Dattaw Hill
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Sanidine] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
        • Pein-Pyit (Painpyit; Pyan Pyit)
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
Ted Themelis (2008) Gems & mines of Mogok
[var: Moonstone] Ted Themelis (2008) Gems & mines of Mogok
Madagascar
 
  • Tuléar Province (Toliara)
    • Anosy Region (Fort Dauphin Region)
      • Betroka District
        • Mahasoa East Commune
[var: Sanidine]
[var: Sanidine]
Sri Lanka
 
  • Sabaragamuwa Province
    • Ratnapura District
[var: Moonstone] Gemexplorer.org
[var: Adularia]
USA
 
  • California
    • El Dorado Co.
      • Mount Tallac
        • Fallen Leaf Lake
          • South side of Fallen Leaf Lake
[var: Adularia] Rogers, Austin Flint (1912), Notes on rare minerals from California: Columbia University, School of Mines Quarterly: 33: 376; Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 184.
Zimbabwe
 
  • Mashonaland West
Gemstones, Arthur Thomas, 2008, p.147
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