General Information
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Chemical Formula: |
KAlSi3O8 |
Composition: |
Molecular Weight = 278.33 gm |
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Potassium 14.05 % K 16.92 % K2O |
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Aluminium 9.69 % Al 18.32 % Al2O3 |
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Silicon 30.27 % Si 64.76 % SiO2 |
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Oxygen 45.99 % O |
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______ ______ |
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100.00 % 100.00 % = TOTAL OxIDE |
Empirical Formula: |
KAlSi3O8 |
Environment: |
Intrusive and extrusive igneous, and metamorphic rocks. |
IMA Status: |
Approved IMA 1960 |
Locality: |
Common world wide occurrences. Madagascar |
Name Origin: |
From the Greek orthos - "right" and kalo -" I cleave" in allusion to the mineral's right angle of good cleavage. |
Synonym: |
Adularia |
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Orthose |
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Orthoclase Images
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Prismatic bi-terminated crystal of orthoclase |
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Click on the picture to go to Orthoclase crystal. |
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© Image by A. Darbellay |
Crystallography
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Axial Ratios: |
a:b:c =0.6636:1:0.5534 |
Cell Dimensions: |
a = 8.625, b = 12.996, c = 7.193, Z = 4; beta = 116.016ø V = 724.57 Den(Calc)= 2.55 |
Crystal System: |
Monoclinic-Prismatic H-M Symbol (2/m) Space Group: C2/m |
x Ray Diffraction: |
By Intensity(I/Io): 3.18(1)4.02(0.9) 3.8(0.8) |
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Physical Properties
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Cleavage: |
[001] Perfect, [010] Good |
Color: |
colorless, greenish, grayish yellow, white, or pink. |
Density: |
2.56 |
Diaphaniety: |
Transparent to translucent |
Fracture: |
Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern. |
Habits: |
Prismatic - Crystals Shaped like Slender Prisms (e.g. tourmaline)., Massive - Granular - Common texture observed in granite and other igneous rock., Blocky - Crystal shape tends to be equant (e.g. feldspars). |
Hardness: |
6 - Orthoclase |
Luminescence: |
None. |
Luster: |
Vitreous (Glassy) |
Streak: |
white |
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Optical Properties
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Optical Data: |
Biaxial (-), a=1.518, b=1.522, g=1.523-1.524, bire=0.0050-0.0060, 2V(Calc)=52-70, 2V(Meas)=65-75. Dispersion relatively strong. |
Pleochroism (x): |
colorless. |
Pleochroism (y): |
colorless. |
Pleochroism (z): |
colorless. |
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Classification
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Dana Class: |
Tectosilicate Al-Si Framework |
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with Al-Si frameworks |
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(Na,Ba) feldspars |
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- Orthoclase KAlSi3O8 C2/m 2/m
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- Sanidine (K,Na)(Si,Al)4O8 C2/m 2/m
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- Hyalophane (K,Ba)Al(Si,Al)3O8 C2/m 2/m
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Celsian BaAl2Si2O8 I21/c 2/m |
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- Microcline KAlSi3O8 C1- -1
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- Anorthoclase (Na,K)AlSi3O8 C1- -1
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- Rubicline ! (Rb,K)AlSi3O8 P1- -1
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Strunz Class: |
Silicates |
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ectosilicates (network) without anions unfamiliar to the tetraheders |
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STRUNZ - Silicates [Tectosilicates (network) without anions unfamiliar to the tetraheders {Feldspar group, Buddingtonite - Orthocl |
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- Buddingtonite (NH4)AlSi3O8·0.5(H2O) P21 or P21/m Mono
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- Sanidine (K,Na)(Si,Al)4O8 C2/m 2/m
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Microcline KAlSi3O8 C1- -1 |
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- Orthoclase KAlSi3O8 C2/m 2/m
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- Celsian BaAl2Si2O8 I21/c 2/m
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- Hyalophane (K,Ba)Al(Si,Al)3O8 C2/m 2/m
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- Paraselsian BaAl2Si2O8 P21/c 2/m
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- Slawsonite (Sr,Ca)Al2Si2O8 P21/a 2/m
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