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Orthoclase

A valid IMA mineral species - grandfathered
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About OrthoclaseHide

Formula:
K(AlSi3O8)
Colour:
Colorless to white, greenish white, grayish yellow, pale pink
Lustre:
Vitreous, Sub-Vitreous, Resinous
Hardness:
6
Specific Gravity:
2.55 - 2.63
Crystal System:
Monoclinic
Name:
Named "orthose" in 1801 by Rene Just Haüy from the Greek orthos - "right" 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 orthoklas by Johann Friedrich August Breithaupt.
Feldspar Group. K Feldspar subgroup. Celsian-Orthoclase Series.

An intermediate, partially ordered member of the K-feldspar structural series, between disordered, monoclinic sanidine and completely ordered microcline. Because of its intermediate nature, it’s species status could be questioned, but it has grandfather status.

Although petrologists have long used microscopic twinning and other optical properties as a method of distinguishing orthoclase from microcline and sanidine, Wright and Stewart (1958) used the position of certain d values in XRD patterns to calculate the degree of disorder of a K-feldspar (plus it’s Na content) and thus have a more quantitative method to identify its structural state.

Sanidine forms in rapidly cooled volcanic rocks (eg. Rhyolite) and some high level intrusive dykes, whilst microcline forms in more slowly cooled rocks like granite pegmatites. Orthoclase is the most common member of the series, being the dominant K-feldspar in most granite, granitic rocks and other plutonic rocks, with an intermediate cooling regime. Despite the enormous number of reports of orthoclase in granite pegmatites, orthoclase is rare in pegmatitic pockets. It has, however, been known to be stabilised by structural defects in some pegmatites (eg. Liu et al., 2018). Partially ordered, monoclinic orthoclase may persist metastably at lower temperatures (Prince et al., 1973). It constitutes some hydrothermal “adularia” (some is microcline).




Unique IdentifiersHide

Mindat ID:
3026
Long-form identifier:
mindat:1:1:3026:3

IMA Classification of OrthoclaseHide

Approved, 'Grandfathered' (first described prior to 1959)
IMA Formula:
KAlSi3O8

Classification of OrthoclaseHide

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

Mineral SymbolsHide

As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.

Please only use the official IMA–CNMNC symbol. Older variants are listed for historical use only.

SymbolSourceReference for Standard
OrIMA–CNMNCWarr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43
OrKretz (1983)Kretz, R. (1983) Symbols of rock-forming minerals. American Mineralogist, 68, 277–279.
OrSiivolam & Schmid (2007)Siivolam, J. and Schmid, R. (2007) Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks: List of mineral abbreviations. Web-version 01.02.07. IUGS Commission on the Systematics in Petrology. download
OrWhitney & Evans (2010)Whitney, D.L. and Evans, B.W. (2010) Abbreviations for names of rock-forming minerals. American Mineralogist, 95, 185–187 doi:10.2138/am.2010.3371
OrThe Canadian Mineralogist (2019)The Canadian Mineralogist (2019) The Canadian Mineralogist list of symbols for rock- and ore-forming minerals (December 30, 2019). download
OrWarr (2020)Warr, L.N. (2020) Recommended abbreviations for the names of clay minerals and associated phases. Clay Minerals, 55, 261–264 doi:10.1180/clm.2020.30

Pronunciation of OrthoclaseHide

Pronunciation:
PlayRecorded byCountry
Jolyon RalphUnited Kingdom

Physical Properties of OrthoclaseHide

Vitreous, Sub-Vitreous, Resinous
Transparency:
Transparent, Translucent
Comment:
Slightly pearly on cleavage
Colour:
Colorless to white, greenish white, grayish yellow, pale pink
Streak:
White
Hardness:
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)

Optical Data of OrthoclaseHide

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
Based on recorded range of RI values above.

Interference Colours:
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.

Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.

Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.

Surface Relief:
Low (negative)
Relative to Canada balsam mounting medium (n ≈ 1.537).

This shows the grain boundary and Becke line effect under plane-polarised light, based on the contrast between this mineral's average refractive index and the mounting medium. It does not take into account mineral colouration.
In focus
Interference Figure:
This shows the idealized biaxial acute bisectrix (Bxa) interference figure - the conoscopic view for a grain cut perpendicular to the acute bisectrix, using this mineral's 2V. The two small white dots mark the melatopes - the points where the two optic axes emerge - and are shown only when they fall within the field of view. The coloured bands are isochromatics, and the dark bands are isogyres.

Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
Dispersion:
r > v distinct
Optical Extinction:
X^a = 6°-14°, Y^c = -13° to 21°, Z = b
Pleochroism:
Non-pleochroic

Chemistry of OrthoclaseHide

Mindat Formula:
K(AlSi3O8)
Element Weights:
Element% weight
O45.987 %
Si30.272 %
K14.047 %
Al9.694 %

Calculated from ideal end-member formula.
O
Si
K
Al
Common Impurities:
Na,Fe,Ba,Rb,Ca

Crystallography of OrthoclaseHide

Crystal System:
Monoclinic
Class (H-M):
2/m - Prismatic
Space Group:
B2/m
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 V:
724.57 ų
Z:
4
Morphology:
Short prismatic
Twinning:
Common as Carlsbad, Baveno and Manebach.

Crystallographic forms of OrthoclaseHide

Crystal Atlas:
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Orthoclase no.49 - {110} - Goldschmidt (1913-1926)
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Orthoclase no.50 - {110} - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.58 - {110} - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.64 - Goldschmidt (1913-1926)
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Orthoclase no.65 - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.76 - {010} - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.145 - {110} - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.174 - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.410 - {010} - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.434 - Goldschmidt (1913-1926)
View 3D crystal model
Orthoclase no.460 - {001} - Goldschmidt (1913-1926)
3d models and HTML5 code kindly provided by www.smorf.nl.

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IDSpeciesReferenceLinkYearLocalityPressure (GPa)Temp (K)
0020542OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0293
0020541OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0293
0020540OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0293
0020509OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0293
0020549OrthoclaseKimata M, Saito S, Shimizu M, Iida I, Matsui T (1996) Low-temperature structures of orthoclase and sanidine Neues Jahrbuch fur Mineralogie, Abhandlungen 171 199-2131996Itrongay, Madagascar0293
0020546OrthoclaseGering E (1985) Silizium/Aluminium-Ordnung und Kristallperfektion von Sanidinen Dissertation Kernforshungszentrum Karlsruhe 1 1-971985Madagascar0293
0019651OrthoclaseViswanathan K, Kielhorn H M (1983) Al,Si distribution in a ternary (Ba,K,Na)-feldspar as determined by crystal structure refinement American Mineralogist 68 122-1241983Yugoslavia0293
0020552OrthoclaseDal Negro A, De Pieri R, Quareni S (1978) The crystal structures of nine K feldspars from the Adamello Massif (Northern Italy) Acta Crystallographica B34 2699-27071978Adamello Massif, Northern Italy0293
0020551OrthoclaseDal Negro A, De Pieri R, Quareni S (1978) The crystal structures of nine K feldspars from the Adamello Massif (Northern Italy) Acta Crystallographica B34 2699-27071978Adamello Massif, Northern Italy0293
0020550OrthoclaseDal Negro A, De Pieri R, Quareni S (1978) The crystal structures of nine K feldspars from the Adamello Massif (Northern Italy) Acta Crystallographica B34 2699-27071978Adamello Massif, Northern Italy0293
0020553OrthoclaseFenn P M, Brown G E (1977) Crystal structure of a synthetic, compositionally intermediate, hypersolvus alkali feldspar: evidence for Na, K site ordering Zeitschrift fur Kristallographie 145 124-1451977Synthetic0293
0000320OrthoclasePrince E, Donnay G, Martin R F (1973) Neutron diffraction refinement of an ordered orthoclase structure American Mineralogist 58 500-5071973Himalaya mine, California, USA0293
0000313OrthoclasePhillips M W, Ribbe P H (1973) The structures of monoclinic potassium-rich feldspars American Mineralogist 58 263-2701973St. Gotthard, Switzerland0293
0000167OrthoclaseColville A A, Ribbe P H (1968) The crystal structure of an adularia and a refinement of the structure of orthoclase American Mineralogist 53 25-371968St. Gotthard, Switzerland, known as Spencer C0293
0000166OrthoclaseColville A A, Ribbe P H (1968) The crystal structure of an adularia and a refinement of the structure of orthoclase American Mineralogist 53 25-371968St. Gotthard, Switzerland, known as Spencer B, variety adularia0293
0020547OrthoclaseJones J B, Taylor W H (1961) The structure of orthoclase Acta Crystallographica 14 443-4561961Mogok, Upper Burma0293
0020548OrthoclaseKimata M, Saito S, Shimizu M, Iida I, Matsui T (1996) Low-temperature structures of orthoclase and sanidine Neues Jahrbuch fur Mineralogie, Abhandlungen 171 199-2131996Itrongay, Madagascar0121
0020543OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0600
0020544OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown0900
0020545OrthoclaseAngel R J, Ross N L, Zhao J, Sochalski-Kolbus L, Kruger H, Schmidt B C (2013) Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars European Journal of Mineralogy 25 597-6142013Unknown01075
0007930OrthoclaseTseng H-Y, Heaney P J, Onstott T C (1995) Characterization of lattice strain induced by neutron irradiation Physics and Chemistry of Minerals 22 399-40519950293
0007929OrthoclaseTseng H-Y, Heaney P J, Onstott T C (1995) Characterization of lattice strain induced by neutron irradiation Physics and Chemistry of Minerals 22 399-40519950293
0007928OrthoclaseTseng H-Y, Heaney P J, Onstott T C (1995) Characterization of lattice strain induced by neutron irradiation Physics and Chemistry of Minerals 22 399-40519950293
0018075OrthoclaseChao S, Hargreaves A, Taylor W (1940) The structure of orthoclase. _cod_database_code 1011205 Mineralogical Magazine 25 498-51219400293
CIF Raw Data - click here to close

X-Ray Powder DiffractionHide

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

Geological EnvironmentHide

Paragenetic Mode(s):
Geological Setting:
Common feldspar of high-temperature granites, syenites, high-grade metamorphic rocks, and some felsic extrusive rocks, eg some rhyolites (although these tend to contain sanidine when fresh).

Synonyms of OrthoclaseHide

Other Language Names for OrthoclaseHide

Varieties of OrthoclaseHide

AglauriteA variety of Orthoclase feldspar with a blue reflection.
Originally described from Teplice (Teplitz), Ústí Region, Bohemia (Böhmen; Boehmen), Czech Republic.
Arsenic-bearing OrthoclaseAs-bearing orthoclase variety from Tolbachik volcano (Arsenatnaya and Yadovitaya fumaroles). Compare filatovite.
Barium-OrthoclaseA Ba-bearing orthoclase with K:Ba < 6.
Barium-bearing Orthoclase
DelawariteA variety of orthoclase with a pearly lustre.
Na-orthoclaseDescribed as a Na- rich orthoclase or cryptoperthite; possibly actually Sanidine.
ParadoxiteAn obsolete name for flesh-coloured orthoclase (adularia) occurring in hydrothermal veins, originally described from the tin mines near Marienberg, Saxony, Germany by A. Breithaupt in 1866 (Berg. Hütt., xxv, 35. as 'Paradoxit')

See discussion: https://w...
PetrosilexA historical term for a compact feldspar, see also orthoclase felsite.
Rainbow Lattice SunstoneA variety of orthoclase with unusual crystallographically-oriented inclusions of magnetite, hematite and biotite. See also sunstone.

Relationship of Orthoclase to other SpeciesHide

Other Members of K Feldspar:
MicroclineK(AlSi3O8)Tric. 1
SanidineK(AlSi3O8)Mon. 2/m : B2/m
Forms a series with:

Common AssociatesHide

Associations Based on Photo Data:
654 photos of Orthoclase associated with QuartzSiO2
416 photos of Orthoclase associated with 'Smoky Quartz'SiO2
258 photos of Orthoclase associated with AlbiteNa(AlSi3O8)
190 photos of Orthoclase associated with AegirineNaFe3+Si2O6
185 photos of Orthoclase associated with SchorlNaFe2+3Al6(Si6O18)(BO3)3(OH)3(OH)
183 photos of Orthoclase associated with MuscoviteKAl2(AlSi3O10)(OH)2
177 photos of Orthoclase associated with FluoriteCaF2
119 photos of Orthoclase associated with 'Aquamarine'
117 photos of Orthoclase associated with FluorapatiteCa5(PO4)3F
116 photos of Orthoclase associated with Epidote(CaCa)(AlAlFe3+)O[Si2O7][SiO4](OH)

Related Minerals - Strunz-mindat GroupingHide

9.FA.BonaccorsiiteKK2Na3(Al6Si36)O84Hex. 6/mmm(6/m2/m2/m) : P6/mcc
9.FA.HexacelsianBaAl2Si2O8Hex. 6/mmm(6/m2/m2/m) : P63/mcm
9.FA.Wodegongjieite KCa3(Al7Si9)O32Hex. 6/mmm(6/m2/m2/m) : P6/mcc
9.FA.05Panunzite(K,Na)AlSiO4Hex. 6 : P63
9.FA.05Yoshiokaite(Ca,Na)[Al(Al,Si)O4]Trig. 3 : P3
9.FA.05NephelineNa3K(Al4Si4O16)Hex. 6 : P63
9.FA.05TrinephelineNaAlSiO4Hex. 6 : P61
9.FA.05Davidsmithite(Ca,◻)2Na6Al8Si8O32Hex. 6 : P63
9.FA.05KaliophiliteKAlSiO4Trig. 3 : P3
9.FA.05KalsiliteKAlSiO4Hex. 622 : P6322
9.FA.05'Carnegieite'NaAlSiO4Tric.
9.FA.05MegakalsiliteKAlSiO4Hex. 6 : P63
9.FA.05TrikalsiliteK2NaAl3(SiO4)3Hex. 6 : P63
9.FA.10MalinkoiteNaBSiO4Hex. 6 : P63
9.FA.15VirgiliteLiAlSi2O6Hex. 622 : P6222
9.FA.25LisitsyniteKBSi2O6Orth. 222 : P222
9.FA.30FerrisanidineK[Fe3+Si3O8]Mon. 2/m : B2/m
9.FA.30Buddingtonite(NH4)(AlSi3O8)Mon. 2 : P21
9.FA.30RubiclineRb(AlSi3O8)Mon. 2/m : B2/m
9.FA.30'Monalbite'NaAlSi3O8Mon. 2/m : B2/m
9.FA.30MicroclineK(AlSi3O8)Tric. 1
9.FA.30 va'Germanate-celsian'BaAl2Ge2O8
9.FA.30CelsianBa(Al2Si2O8)Mon. 2/m
9.FA.30SanidineK(AlSi3O8)Mon. 2/m : B2/m
9.FA.35ReedmergneriteNaBSi3O8Tric. 1 : P1
9.FA.35AlbiteNa(AlSi3O8)Tric. 1
9.FA.35AnorthiteCa(Al2Si2O8)Tric. 1 : P1
9.FA.40ParacelsianBa(Al2Si2O8)Mon. 2/m : P21/b
9.FA.45SvyatoslaviteCa(Al2Si2O8)Mon. 2 : P21
9.FA.45KumdykoliteNa(AlSi3O8)Orth. mmm(2/m2/m2/m) : Pnnm
9.FA.50SlawsoniteSr(Al2Si2O8)Mon. 2/m : P21/b
9.FA.55LisetiteCaNa2Al4Si4O16Orth. mm2
9.FA.60StronalsiteNa2SrAl4Si4O16Orth.
9.FA.60BanalsiteNa2BaAl4Si4O16Orth. mm2 : Iba2
9.FA.65MaleeviteBaB2Si2O8Orth. mmm(2/m2/m2/m) : Pnma
9.FA.65PekoviteSrB2Si2O8Orth. mmm(2/m2/m2/m) : Pnma
9.FA.65DanburiteCaB2Si2O8Orth. mmm(2/m2/m2/m)
9.FA.70LiebermanniteKAlSi3O8Tet. 4/m : I4/m
9.FA.70LinguniteNaAlSi3O8Tet. 4/m : I4/m
9.FA.70StöffleriteCaAl2Si2O8Tet. 4/m : I4/m
9.FA.75PfaffenbergiteKNa3(Al4Si12)O32Hex. 6/mmm(6/m2/m2/m) : P6/mcc
9.FA.75KokchetaviteK(AlSi3O8)Hex. 6/mmm(6/m2/m2/m) : P6/mcc

RadioactivityHide

Radioactivity:
Element % Content Activity (Bq/kg) Radiation Type
Uranium (U) 0.0000% 0 α, β, γ
Thorium (Th) 0.0000% 0 α, β, γ
Potassium (K) 14.0474% 4,355 β, γ

For comparison:

  • Banana: ~15 Bq per fruit
  • Granite: 1,000–3,000 Bq/kg
  • EU exemption limit: 10,000 Bq/kg

Note: Risk is shown relative to daily recommended maximum exposure to non-background radiation of 1000 µSv/year. Note that natural background radiation averages around 2400 µSv/year so in reality these risks are probably extremely overstated! With infrequent handling and safe storage natural radioactive minerals do not usually pose much risk.

Interactive Simulator:

Note: The mass selector refers to the mass of radioactive mineral present, not the full specimen, also be aware that the matrix may also be radioactive, possibly more radioactive than this mineral!

Activity:

DistanceDose rateRisk
1 cm
10 cm
1 m

The external dose rate (D) from a radioactive mineral is estimated by summing the gamma radiation contributions from its Uranium, Thorium, and Potassium content, disregarding daughter-product which may have a significant effect in some cases (eg 'pitchblende'). This involves multiplying the activity (A, in Bq) of each element by its specific gamma ray constant (Γ), which accounts for its unique gamma emissions. The total unshielded dose at 1 cm is then scaled by the square of the distance (r, in cm) and multiplied by a shielding factor (μshield). This calculation provides a 'worst-case' or 'maximum risk' estimate because it assumes the sample is a point source and entirely neglects any self-shielding where radiation is absorbed within the mineral itself, meaning actual doses will typically be lower. The resulting dose rate (D) is expressed in microsieverts per hour (μSv/h).

D = ((AU × ΓU) + (ATh × ΓTh) + (AK × ΓK)) / r2 × μshield

Fluorescence of OrthoclaseHide

May fluoresce dull white or red in SW UV

Other InformationHide

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

Orthoclase in petrologyHide

Internet Links for OrthoclaseHide

References for OrthoclaseHide

Reference List:

Localities for OrthoclaseHide

Showing 3,508 localities.

This 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.
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