Mineral Information and Data:

Anorthoclase
(Na,K)AlSi3O8 [CNMNC approved formula]

Named in 1885 from the Greek "an" = not, "orthos" = right (angle), and "klan" = to cleave.

IMA status : Intermediate Species Member; 1885 ()
CLASSIFICATION 
Dana
(8th edition) :
76.01.01.06  
  (76) Tectosilicate Al-Si Framework
  (76.01) with Al-Si frameworks
  (76.01.01) K (Na,Ba) feldspars
  
Nickel-Strunz
(10th edition) :
09.FA.30
  (09) Silicates
  (09.F) Tektosilicates without Zeolitic H2O
  (09.FA) Tektosilicates without additional non-tetrahedral anions
  
Crystal system: Triclinic System
Point group (H-M): 1 — pinacoidal
Unit cell: a = 8.29 Ǻ, b = 12.97 Ǻ, c = 7.16 Ǻ, a = 91.1°, b = 116.3°, g = 90.2°
Crystal Habit: Crystals usually short prismatic, blocky, tabular; massive, cleavable, granular
Twinning: Common by Carlsbad, Baveno or Manebach law; also by Albite or Pericline law
  
Color: Colorless, white, grey, yellowish, reddish, greenish
Diaphaneity: Transparent
Luster: Vitreous, somewhat pearly on cleavages
Hardness (Mohs): 6 to 6.5
Measured Density: 2.56 to 2.6 g/cm3
Cleavage: 5; {001} and {010} perfect, {100, }{110}, {201}partings
Tenacity: Brittle
Fracture: Uneven
Streak: White
  
Geologic Setting: In high-temperature sodic volcanic and hypabyssal rocks
Mineral Association: Typically in a fine-grained groundmass or weathered out as loose crystals
  
Other Properties: Gemstone Unlikely Available.
Comments: Feldspar (alkali) group; intermediate between low sanidine and high albite.

 

Anorthoclase
Anorthoclase in matrix
Origin: Kusukidani, Wakayama Prefecture, Kinki Region, Honshu island, Japan
Sample size: 1.3 x 0.9 x 0.7 cm
Photo: Rob Lavinsky

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