Silicates: Sillimanite
Diagnostic Card.
On the picture. Elongated prismatic crystals of sillimanite.
Al 2 O SiO 4
Crystal system orthorhombic
Hardness 6.5-7.5
Specific gravity 3.23-3.27
Cleavage is easily seen
The break in the wrong
Colour colorless, differently colored
Color white powder
Shine from the glass to the silky
Sillimanite - aluminum silicate. Silky gloss, glass. Transparent. Colours: gray, yellow, green, brown. The bar is white. Fracture is uneven. Cleavage is perfect. Crystals (orthorhombic system) are rare. More common columnar or fibrous aggregates. It forms a close fusion with quartz fiber structure. Distribution Area: Spessart (Germany), Tyrol (Austria).
Sillimanite crystallizes in the orthorhombic system, in the form of elongated prisms of square cross section, with vertical hatching. Enough common form dense masses, quite remarkable because of its fiber (fiberboard) or columnar structure. The individual fibers may be encountered inclusions of quartz and other minerals. Being aplohromaticheskim (capable of receiving a different color), sillimanite may be colorless, white, light gray and painted in a variety of yellow, brown and green tones.
Chemical composition. Alumina (A1g0z) 63.1% silica dioxide (SiOz) 36,9%, is sometimes present as an impurity (3.2%) of iron oxide. Form crystalline precipitates. Small needles or entangled fibrous formation, sometimes microscopic fine hairs. Class symmetry. Rombo-bipyramidal - mmm. Cleavage. Perfect parallel to the c axis, with respect to (010). Aggregates. Fibrous, puchkoobraznye.
The mineral is transparent and translucent, its brilliance is modified from the glass to the bold, while the fibrous varieties - to silky. Sillimanite - hard and heavy mineral that has manifested easy cleavage along the extension, while a break in the perpendicular direction - wrong. This mineral has a chemical composition identical to andalusite and kyanite - but they crystallize in different crystal system, presenting a classic example of polymorphism.
Diagnostic features.
This mineral form are not soluble in acids and does not melt in the flame of a candle. Behavior in acids. Insoluble.
Origin.
It occurs mainly in rocks that were formed as a result of regional metamorphism at elevated temperatures and pressures. It is also present in some species pegmatite.
Place of Birth.
Present in pegmatites in the Vallone del Nibbio near the town of Mergozzo (Verbania province) and in other places nearby in the form of long crystals of greenish-gray color. In the form of beams it is also found in the area of Sondalo (Sondrio Province). It is worth mentioning the location Brissago (Switzerland), License text ridge in the Austrian Tirol Alps, Bodenmais (Bavaria, Germany), Ponzhibo (France) and Mogok (Burma). In Sri Lanka extract highly valued stones transparent sapphire-blue color, suitable for jewelry use. In Brazil and the United States of alluvium extracted kind of sillimanite fiberboard.
Application.
From major Indian deposits of sillimanite is produced on an industrial scale as a raw material for the production of refractory materials.
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