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Anorthite

Discovered in 1823;   IMA status:  Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

CaAl₂SiO₈

Calcium Aluminum Silicate

Molecular Weight:

277.41 gm

Composition:

Sodium

0.41 %

Na

0.56 %

Na₂O

Calcium

13.72 %

Ca

19.20 %

CaO

Aluminum

18.97 %

Al

35.84 %

Al₂O₃

Silicon

20.75 %

Si

44.40 %

SiO₂

Oxygen

46.14 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/J.07-70

Nickel-Strunz 10th Ed. ID:

9.FA.35

9 : SILICATES (Germanates)
F : Tektosilicates without zeolitic H₂O
A : Tektosilicates without additional non-tetrahedral anions

Related to:

Feldspar Group. Plagioclase Series. Albite - Anorthite Series. Trimorphous with Dmisteinbergite and Svyatoslavite; low- and high-temperature structural modifications are recognized.

Members of Group:

Feldspar Group: Albite, Andesine, Anorthoclase, Banalsite, Buddingtonite, Bytownite, Celsian, Dmisteinbergite, Hyalophane, Labradorite, Microline, Oligoclase, Orthoclase, Paracelsian, Reedmergnerite, Sanidine, Slawsonite, Stronalsite, Svyatoslavite

Varieties:

Barium-Anorthite (of Nockolds and Zies), Bytownite, Labradorite, Lindsayite, Lynx Eye

Synonyms:

Amphodelite, Anorthoîte, Barsovite, Beffanite, Beffonite, Biotine, Calciclase, Calcium Feldspar, Christianite, Cyclopite, Didymolite, Indianite, Latrobite, Lepolite, Lime-Feldspar, Thiorsauite

Crystal Data

Crystallography:

Triclinic - Pinacoidal

Crystal Habit:

Crystals commonly short, prismatic, to 2 cm; lamellar, coarse granular, massive.

Twinning:

Commonly polysynthetic on the Albite law; also after the Pericline, Carlsbad, Manebach, and Baveno laws.

Physical Properties

Cleavage:

Perfect on {001}, less so on {010}, Imperfect on {110}

Fracture:

Conchoidal to Uneven

Tenacity:

Brittle

Moh's Hardness:

6.0 - 6.5

Density:

2.74 - 2.76 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

White, Grayish, Reddish; Colorless in thin section

Transparency:

Translucent, Opaque; rarely Transparent

Luster:

Vitreous

Refractive Index:

1.573 - 1.590  Biaxial ( - )

Birefringence:

0.0110 - 0.0120

Dispersion:

Weak, r < v

Pleochroism:

X = colorless, Y = colorless, Z = colorless

Occurances

Geological 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.

Common Associations:

Olivine, Pyroxene, Corundum

Common Impurities:

Ti, Fe, Na, K

Type Locality:

Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy

Year Discovered:

1823

View mineral photos:

Anorthite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Anorthite is the rarest member of the Plagioclase Feldspars of the Feldspar Group of minerals that includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. The Plagioclase Feldspars form a series between Albite and Anorthite. Albite is the sodium-rich end member while Anorthite is the calcium-rich end member and Oligoclase is the intermediate member. Faceted Anorthite gems are almost always small and although Anorthite is a widely distributed rock-forming mineral, gem quality crystals are quite rare.

Anorthite was named in 1823 by Gustav Rose from the Greek words αν and ορθός, meaning not right angle or oblique, in allusion to the oblique triclinic form of the crystals.

Sources of gem quality Anorthite crystals are found at Ettringer Bellerberg Mt., Eifel Mts., Rhineland-Palatinate, Germany; the classic location of Monte Somma, Naples Province, Campania, Italy; Miyake Island, Izu Archipelago, Tokyo Prefecture, Japan and in the US at Iron Mountain No. 2 District, Sierra County, New Mexico.

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