Mineral Information and Data:

CaAl2Si2O8 [CNMNC approved formula]

Named from the Greek an = not and orthos = right (angle), for its triclinicity.

IMA status : Valid species (Pre-IMA 1823) Grandfathered
(8th edition) :  
  (76) Tectosilicate Al-Si Framework
  (76.01) with Al-Si frameworks
  (76.01.03) Plagioclase series
(10th edition) :
  (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 = 8.1768Å, b = 12.8768Å, c = 14.169Å α = 93.17°, β = 115.85°, γ = 92.22°
Crystal Habit: Crystals usually short prismatic; massive, cleavable
Twinning: Common by Albite law; also by Carlsbad, Baveno, Manebach, or Pericline law
Color: Colorless, white, greyish, reddish
Diaphaneity: Transparent to translucent
Luster: Vitreous
Hardness (Mohs): 6 to 6.5
Measured Density: 2.74 to 2.76 g/cm3
Cleavage: 3; {001} perfect, {010} nearly perfect, {110} imperfect
Tenacity: Brittle
Fracture: Conchoidal to uneven
Streak: White
Polymorphism and Series: Trimorphous with dmisteinbergite and svyatoslavite; low- and high-temperature structural modifications are recognized.
Geologic Setting: A rare constituent of mafic plutonic and volcanic rocks. In some granulite facies
metamorphic rocks; in metamorphosed carbonate rocks; with corundum deposits. Known from meteorites.
Mineral Association: Olivine, pyroxene, corundum
Synonyms/varieties: Plagioclase [An 90-100]
Other Properties: Gemstone Unlikely Available.
Comments: End member of the plagioclase series, feldspar group.


Origin: Miakajima Island, Japan
Sample size: 23 x 18 x 20 mm
Photo: Dan Weinrich


From volcano eruption in 1870. These crystals are opaque white coated with a film of reddish or blackish glassy lava
Origin: Miyake Island (Miyake-jima), Izu Archipelago, Tokyo Prefecture, Japan
Sample size: 1.6 x 1.2 x 1 cm
Photo: Rob Lavinsky