Andalusite
Andalusite is strongly pleochroic showing colours of olive green to brick red. Its strong pleochroism is extremely distinctive and extremely attractive as a treasure that is faceted. Andalusite trimorphous with Kyanite and Sillimanite. Trimorphous identifies a mineral that shares the chemistry that is same an alternate crystal framework with two other minerals. Andalusite has two other varieties that are interesting. One is called Chiastolite. It will always be opaque and contains black carbon inclusions that 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 grey or background that is brown. The name Chiastolite is from the Greek chiastic meaning arranged diagonally because the pattern of carbon inclusions resembles the Greek letter Chi, which will be written X. One other selection of Andalusite is Viridine which can be a rich, grass colour that is green to its manganese content. Viridine gets its title through the Latin term Viridis for green. A source that is current of Andalusite crystals is the Santa Teresa region, Espirito Santo, Brazil. Chiastolite and Viridine are also present in Brazil.
Andalusite is widespread. Some localities for good crystals follow. From Hornachuelos, near Córdoba, Córdoba Province, Spain. Regarding the Lisens Alp, Selraintal, Tirol, Austria. At Gefrees, Bodenmais, and somewhere else 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, Southern Australia. At Nawalapitiya, Sri Lanka. Gem crystals from the Santa Teresa region, Espírito Santo, Brazil.
| 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 |
| 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} |
| 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 |
| Colour: | 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 |
| Ultraviolet Fluorescence : |
Non-Fluorescent |
