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Orthoclase

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Formula:
KAlSi3O8
System:
Monoclinic
Colour:
Colorless to white, ...
Hardness:
6
Member of:
Name:
Named "orthose" in 1801 by Rene Just Haüy from the Greek orthos - "right" and kalo - "I cleave" in allusion to the mineral's right angle of good cleavage. The sense of Haüy's name was that the mineral was a feldspar, but he did not specify a type-locality, nor did Haüy give a chemical analysis. The name was changed in 1823 to orthoklase by Johann Friedrich August Breithaupt. Although petrologists had used twinning as a method of distinguishing orthoclase from microcline, Wright and Stewart (1958) used the difference in certain d values to calculate a degree of disorder of a sample and thus have a quantitative method to identify the structural state of a potassium feldspar. Despite the enormous number of reports of orthoclase in granite pegmatites, orthoclase is extremely rare in such deposits.
Polymorph of:

Classification of Orthoclase

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
9.FA.30

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H2O
A : Tektosilicates without additional non-tetrahedral anions
Dana 7th ed.:
76.1.1.1
76.1.1.1

76 : TECTOSILICATES Al-Si Framework
1 : Al-Si Framework with Al-Si frameworks
16.3.6

16 : Silicates Containing Aluminum and other Metals
3 : Aluminosilicates of K
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Occurrences of Orthoclase

Geological Setting:
Common feldspar of high temperature granites, syenites, high grade metamorphic rocks, and felsic extrusive rocks.

Physical Properties of Orthoclase

Vitreous, Sub-Vitreous, Resinous
Diaphaneity (Transparency):
Transparent, Translucent
Comment:
Slightly pearly on cleavage
Colour:
Colorless to white, Greenish white, Grayish yellow, Pale pink
Streak:
White
Hardness (Mohs):
6
Hardness Data:
Mohs hardness reference species
Tenacity:
Brittle
Cleavage:
Perfect
Perfect on {001}, good on {010}
Parting:
On {100} {110} {110} {201}
Fracture:
Irregular/Uneven, Conchoidal
Density:
2.55 - 2.63 g/cm3 (Measured)    2.563 g/cm3 (Calculated)

Crystallography of Orthoclase

Crystal System:
Monoclinic
Class (H-M):
2/m - Prismatic
Space Group:
B2/m
Space Group Setting:
C2/m
Cell Parameters:
a = 8.5632(11) Å, b = 12.963(14) Å, c = 7.299(11) Å
β = 116.073(9)°
Ratio:
a:b:c = 0.661 : 1 : 0.563
Unit Cell Volume:
V 724.57 ų
Z:
4
Morphology:
Short prismatic
Twinning:
Common as Carlsbad, Baveno and Manebach.

Crystallographic forms of Orthoclase

Crystal Atlas:
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Orthoclase no.49 - Goldschmidt (1913-1926)
Orthoclase no.50 - Goldschmidt (1913-1926)
Orthoclase no.58 - Goldschmidt (1913-1926)
Orthoclase no.64 - Goldschmidt (1913-1926)
Orthoclase no.65 - Goldschmidt (1913-1926)
Orthoclase no.76 - Goldschmidt (1913-1926)
Orthoclase no.145 - Goldschmidt (1913-1926)
Orthoclase no.174 - Goldschmidt (1913-1926)
Orthoclase no.410 - Goldschmidt (1913-1926)
Orthoclase no.434 - Goldschmidt (1913-1926)
Orthoclase no.460 - Goldschmidt (1913-1926)
3d models and HTML5 code kindly provided by www.smorf.nl.

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Transparency
Opaque | Translucent | Transparent

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X-Ray Powder Diffraction:
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Radiation - Copper Kα
Data Set:
Data courtesy of RRUFF project at University of Arizona, used with permission.
X-Ray Powder Diffraction Data:
d-spacingIntensity
4.22 (70)
3.77 (80)
3.47 (45)
3.31 (100)
3.29 (60)
3.24 (65)
2.992 (50)
Comments:
ICDD 19-931

Optical Data of Orthoclase

Type:
Biaxial (-)
RI values:
nα = 1.518 - 1.520 nβ = 1.522 - 1.524 nγ = 1.522 - 1.525
2V:
Measured: 35° to 75°, Calculated: 52° to 70°
Birefringence:
0.004
Max Birefringence:
δ = 0.004 - 0.005
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Low
Dispersion:
r > v distinct
Optical Extinction:
X^a = 6°-14°, Y^c = -13° to 21°, Z = b
Pleochroism:
Non-pleochroic

Chemical Properties of Orthoclase

Formula:
KAlSi3O8
Essential elements:
All elements listed in formula:
Common Impurities:
Na,Fe,Ba,Rb,Ca

Relationship of Orthoclase to other Species

Series:
Forms a series with Hyalophane (see here)
Forms a series with Celsian (see here)
Member of:
Other Members of Group:
AlbiteNaAlSi3O8
Amazonite
Andesine(Na,Ca)[Al(Si,Al)Si2O8]
AnorthiteCaAl2Si2O8
Anorthoclase(Na,K)AlSi3O8
BanalsiteBaNa2[Al2Si2O8]2
Buddingtonite(NH4)AlSi3O8
Bytownite(Ca,Na)[Al(Al,Si)Si2O8]
CelsianBaAl2Si2O8
DmisteinbergiteCaAl2Si2O8
Hyalophane(K,Ba)[Al(Si,Al)Si2O8]
Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
MicroclineKAlSi3O8
Oligoclase(Na,Ca)[Al(Si,Al)Si2O8]
ParacelsianBaAl2Si2O8
ReedmergneriteNa[BSi3O8]
SanidineKAlSi3O8
Slawsonite(Sr,Ca)Al2Si2O8
StronalsiteSrNa2[Al2Si2O8]2
SvyatoslaviteCaAl2Si2O8
9.FA.05KaliophiliteKAlSiO4
9.FA.05KalsiliteKAlSiO4
9.FA.05Nepheline(Na,K)AlSiO4
9.FA.05Panunzite(K,Na)AlSiO4
9.FA.05Trikalsilite(K,Na)AlSiO4
9.FA.05Yoshiokaite(Ca,Na)[Al(Al,Si)O4]
9.FA.05MegakalsiliteKAlSiO4
9.FA.10MalinkoiteNaBSiO4
9.FA.15VirgiliteLiAlSi2O6
9.FA.25LisitsyniteKBSi2O6
9.FA.30AdulariaKAlSi3O8
9.FA.30Anorthoclase(Na,K)AlSi3O8
9.FA.30Buddingtonite(NH4)AlSi3O8
9.FA.30CelsianBaAl2Si2O8
9.FA.30Hyalophane(K,Ba)[Al(Si,Al)Si2O8]
9.FA.30MicroclineKAlSi3O8
9.FA.30SanidineKAlSi3O8
9.FA.30Rubicline(Rb,K)AlSi3O8
9.FA.30MonalbiteNaAlSi3O8
9.FA.35AlbiteNaAlSi3O8
9.FA.35Andesine(Na,Ca)[Al(Si,Al)Si2O8]
9.FA.35AnorthiteCaAl2Si2O8
9.FA.35Bytownite(Ca,Na)[Al(Al,Si)Si2O8]
9.FA.35Labradorite(Ca,Na)[Al(Al,Si)Si2O8]
9.FA.35Oligoclase(Na,Ca)[Al(Si,Al)Si2O8]
9.FA.35ReedmergneriteNa[BSi3O8]
9.FA.40ParacelsianBaAl2Si2O8
9.FA.45SvyatoslaviteCaAl2Si2O8
9.FA.45KumdykoliteNaAlSi3O8
9.FA.50Slawsonite(Sr,Ca)Al2Si2O8
9.FA.55LisetiteCaNa2Al4Si4O16
9.FA.60BanalsiteBaNa2[Al2Si2O8]2
9.FA.60StronalsiteSrNa2[Al2Si2O8]2
9.FA.65DanburiteCaB2Si2O8
9.FA.65MaleeviteBaB2Si2O8
9.FA.65PekoviteSrB2Si2O8
9.FA.70Lingunite(Na,Ca)AlSi3O8
9.FA.75KokchetaviteKAlSi3O8
16.3.1LithositeK6Al4Si8O25 · 2H2O
16.3.2KaliophiliteKAlSiO4
16.3.3KalsiliteKAlSiO4
16.3.4LeuciteK(AlSi2O6)
16.3.5MicroclineKAlSi3O8
16.3.7SanidineKAlSi3O8
16.3.8MuscoviteKAl2(AlSi3O10)(OH)2

Other Names for Orthoclase

Name in Other Languages:

Other Information

May fluoresce dull white or red in SW UV
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Industrial Uses:
Ceramics, abrasives, crushed stone, decorative facing in slabbed rock panels, gem materials

References for Orthoclase

Reference List:
Coombs, D.S. (1954) Ferriferous orthoclase from Madagascar. Mineralogical Magazine: 30: 409-427.
Wright, T. L. and Stewart, D. B. (1968) X-ray and optical study of alkali feldspar, I. Determination of composition and structural state from refined unit-cell parameters and 2Y. American Mineralogist, 53, 38-87
Prince, E., Donnay, G., and Martin, R.F. (1973) Neutron diffraction refinement of an ordered orthoclase structure. American Mineralogist: 58: 500-507.
Foland, K.A. (1974) 40Ar diffusion in homogeneous orthoclase and an interpretation of Ar diffusion in K-feldspar. Geochimica et Cosmochimica Acta: 3: 151-166.
Mineralogical Record (1977): 8: 363.
Eggleton, R.A. and Buseck, P.R. (1980) The orthoclase-microcline inversion: A high resolution transmission electron microscope study and strain analysis. Contributions to Mineralogy and Petrology: 74: 123-133.
Priess, U. (1981) Untersuchungen zur Tief-Hoch-Umwandlung von Fe-haltinge Orthoklas-Kristallen aus Madagaskar. Neues Jahrbuch für Mineralogie, Abhandlungen: 141: 17-29.
Goldsmith, J. (1988) Enhanced Al/Si diffusion in KAlSi3O8 at high pressures: the effect of hydrogen. Journal of Geology: 96: 109-124.
Foland, K.A. and Xu, Y.-P. (1990) Diffusion of 40Ar and 39Ar in irradiated orthoclase. Geochimica et Cosmochimica Acta: 54: 3147-3158.
Widder, W. (1991) Röntgentopographische Untersuchungen am Sanidin von Volkesfeld und Orthoklas von Madagaskar. Diplomarbeit an der Fakultät für Physik, Universität Karlsruhe, Karlsruhe, Germany.
Waldron, K.A., Parsons, I., and Brown, W.L. (1993) Solution-redeposition and the orthoclase-microcline transformation: evidence from granulites and relevance to 18O exchange. Mineralogical Magazine: 57: 687-695.
Acta Crystallographica (1995): A53.
Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1995) Handbook of Mineralogy, Volume II. Silica, Silicates. Mineral Data Publishing, Tucson, AZ, 904pp., 2 sub-volumes: 606.
Kimata, M., Saito, S., Shimizu, M., Iida, I., and Tomoaki, M. (1996) Low-temperature crystal structures of orthoclase and sanidine. Neues Jahrbuch für Mineralogie, Abhandlungen: 171: 199-213.
Villa, I.M. (1996) The influence of irradiation and heating schedule on Ar diffusion in gem-quality orthoclase from Madagascar. Journal Conf. Abstr.: 1(1): 652.
Arnaud, N.O. and Kelley, S.P. (1997) Argon behaviour in gem-quality orthoclase from Madagascar: experiments and some consequences for 40Ar/39Ar geochronology. Geochimica et Cosmochimica Acta: 61: 3227-3255.
Fenter, P., Teng, H., Geissbühler, P., Hanchar, J.M., Nagy, K., and Sturchio, N.C. (2000) Atomic-scale structure of the orthoclase (001) - water interface measured with high-resolution X-ray reflectivity. Geochimica et Cosmochimica Acta: 64: 3663-3673.
Teng, H.H., Fenter, P., Cheng, L., and Sturchio, N.C. (2001) Resolving orthoclase dissolution processes with atomic force microscopy and X-ray reflectivity. Geochimica et Cosmochimica Acta: 65: 3459-3474.

Internet Links for Orthoclase

Specimens:
The following Orthoclase specimens are currently listed for sale on minfind.com.

Localities for Orthoclase

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
Mineral and/or Locality  
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