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Andalusite

 

Andalusite

 

Discovered in 1789;   IMA status:  Valid (pre-IMA; Grandfathered)

 

Chemistry

 

 

Chemical Formula:

Al2SiO5 

 

Aluminum Silicate

Molecular Weight:

162.05 gm

Composition:

Aluminum

33.30 %

Al

62.92 %

Al2O3

 

Silicon

17.33 %

Si

37.08 %

SiO2

 

Oxygen

49.37 %

O

 

 

 

 

100.00 %

 

100.00 %

= TOTAL OXIDE

 

 

Classification

   

   

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/B.02-20

Nickel-Strunz 10th Ed. ID:

9.AF.10

 

9 : SILICATES (Germanates)
A : Nesosilicates
F : Nesosilicates with additional anions; cations in [4], [5] and/or only [6] coordination

Related to:

Andalusite-Kanonaite Series. Trimorphous with Kyanite and Sillimanite. Isostructural with Kanonaite.

Varieties:

Chiastolite,  Titanian Andalusite, Viridine

Synonyms:

Andalousite, Andaluzite, Apyre, Feldspath apyre, Jamesonite (of Leman), Lohestite  

 

 

Crystal Data

   

   

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

As euhedral crystals or columnar aggregates having nearly square cross sections, commonly elongated || [001], to 20 cm. Also fibrous, compact, massive.

Twinning:

Rare on {101}

 

 

Physical Properties

   

 

Cleavage:

Good on {110}, Poor on {100}

Fracture:

Irregular/Uneven, Sub-Conchoidal

Tenacity:

Brittle

Moh's Hardness:

6.5 - 7.5

Density:

3.13 - 3.17 (g/cm3)

Luminescence:

None

Radioactivity:

Not Radioactive

 

 

Optical Properties

   

   

Color:

Pink, Violet, Yellow, Green, White, Gray; Colorless to Pink or Green in thin section.

Transparency:

Transparent to Translucent

Luster:

Vitreous, Sub-Vitreous, Greasy

Refractive Index:

1.629 - 1.650  Biaxial ( - ) or ( + )

Birefringence:

0.009 - 0.010

Dispersion:

Strong; r < v 

Pleochroism:

Distinct; X = reddish pink, Y = greenish yellow, Z = greenish yellow

 

 

Occurances

   

   

Geological Setting:

A result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists. Rare in granites and pegmatites, which however afford the largest crystals. Detrital in some sandstones.

Common Associations:

Kyanite, Sillimanite, Cordierite, Corundum, Garnet, Tourmaline, Mica

Common Impurities:

None

Type Locality:

Ronda Massif, Málaga, Andalusia, Spain

Year Discovered:

1789

View mineral photos:

Andalusite Mineral Photos and Locations

 

 

More Information

   

   

 

Mindat.org
Webmineral.com

 

 

Andalusite is strongly pleochroic showing colors of olive green to brick red. Its strong pleochroism is very distinctive and extremely attractive as a faceted gem. Andalusite trimorphous with Kyanite and Sillimanite. Trimorphous refers to a mineral that shares the same chemistry but a different crystal structure with two other minerals. Andalusite has two other interesting varieties. One is called Chiastolite. It is usually opaque and contains black carbon inclusions which are arranged in regular symmetrical shapes, usually in the form of a cross or X. Chiastolite cut in cross section and polished makes for very unique and attractive cabochons, beads or pendants showing a well-formed black cross on a gray or brown background. The name Chiastolite is from the Greek chiastos meaning arranged diagonally, because the pattern of carbon inclusions resembles the Greek letter Chi, which is written X. The other variety of Andalusite is Viridine which is a rich, grass green color due to its manganese content. Viridine gets its name from the Latin word viridis for green. A current source of fine Andalusite crystals is the Santa Teresa district, Espirito Santo, Brazil. Chiastolite and Viridine are also found in Brazil.

Andalusite was named in 1798 by Jean-Claude Delamétherie for the type locality, Andalusia region, Spain.

Andalusite is widespread. Some localities for good crystals follow. From Hornachuelos, near Córdoba, Córdoba Province, Spain. On the Lisens Alp, Selraintal, Tirol, Austria. At Gefrees, Bodenmais, and elsewhere in Bavaria, Germany. From the Claggau quarry, County Galway, Ireland. In the USA, from Lancaster, Worcester County, Massachusetts; Leiperville, Delaware County, Pennsylvania; around Custer, Custer County, South Dakota; in California, from Fresno, Fresno County, near Ogilby, Cargo Muchacho Mountains, Imperial County, and near Daltons Ranch, Madera County. From Mt. Howden, Bimbowrie, South Australia. At Nawalapitiya, Sri Lanka. Gem crystals from the Santa Teresa district, Espírito Santo, Brazil.

Andalusite gems for sale:

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