Andalusite

ANDALUSITE (from the name of the "Andalucia" historical region within Spain, where it has been found for the first time * EN: andalusite; DE: Andalusit; FR: andalousite; ES: andalucita; RU: андалузит) is the mineral of the sub-class of the island silicates, the polymorphic modification of the silicate of aluminium, AlVI AlV [SiO4]O, which is formed at the lowest pressure and temperature.

Other modifications of the silicate of the same composition are kyanite and sillimanite. The varieties are: viridine is the dark green iron-containing and manganese-containing andalusite (9.6% of Fe2O3, 7% and more of Mn2O3); chiastolite is the opaque andalusite with carbonaceous and other inclusions, which are distributed in such pattern, that they form the dark cross-shaped figure within the cross-section of the crystal. The admixtures are: Fe3+, Ti, Mg, Fe2+, Ca, and others. It crystallizes into the rhombic crystal systems. There are at the foundation of the structure of andalusite the chains of the [AlO6]-octahedra, which are connected with each other by the [SiO4]-tetrahedra and Al atoms with coordination number 5, which is rare for the silicates. It forms the prismatic pseudo-tetragonal crystals, columnar and radiating aggregates, phenocrysts, and other shapes. The colour is pink, brown, yellow, red, green, gray. The transparent crystals have the brightly expressed pleochroism. The cleavage is perfect along the (110) plane. The hardness is 6.5-7.5. The density is 3100-3200 kilograms per cubic metre. During the heating up to 1380 degrees Celsius, it transitions into the mullite. It is the mineral of the zones of metamorphism of the low pressures and high temperature gradient. It is formed during the regional metamorphism, it is characteristic also for the contact-metamorphosed clayish rocks.

It may be found within the schists, hornfels, secondary quartzites, which are rich in alumina, it is known within the granitic pegmatites, quartz veins of the alpine type. It is the terrigenous-clastic component of the sedimentary rocks. It is resistant to weathering, accumulates within the placer deposits. The largest deposit of andalusite White Mountain (USA, California state) is confined to the secondary quartzites. Andalusite is beneficiated by flotation; the collectors are the oleic acid (pH 9), naphthenic oils, and others; the regulator of the medium is soda; it is activated by attritioning, washing, feeding of the water with decreased mineralization; the depressor is the liquid glass. During the separation from the quartz, they conduct the flotation within the acidic medium.

Andalusite is the raw material for the manufacturing of the highly fire-refractory and acid-proof materials, for the obtainment of the alloy of silumin. The transparent pleochroic varieties of andalusite, and also of chiastolite, are the precious stones of the III order, which may be found within India, Sri Lanka, Brazil, Tanzania, Spain, and other countries. The worldwide reserves of andalusite are estimated at 175 million tonnes.