Photo of Sillimanite in Processed & Rough Form
Sillimanite is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of aluminum silicate, their full chemical compound being Al2SiO5.
Sillimanite is a grayish-blue aluminum silicate with an orthorhombic crystal system. It is trimorphic with andalusite and kyanite. All three have the same chemical compound but their atoms each arrange to create three different crystal forms.
It appears as long, slender crystals without distinct terminations, in off-white, gray, brown, pale green, slate-blue, blue-green, and these crystals are often in silky, fibrous aggregates (fibrolite). This fiber sillimanite is mistakenly called "radiated quartz." It also frequently occurs as needle-like crystal inclusions in other minerals, like quartz and feldspars. When it appears as inclusions in rose quartz and other gemstones, it causes that stone to exhibit asterism, star-like rays in reflected light.
It is hard, heavy, with perfect cleavage parallel to its fibers, with uneven fracture. It is transparent to translucent with vitreous to pearly luster. It is insoluble and infusible.
Sillimanite is widespread in high-temperature regional metamorphic rocks and occasionally in pegmatites, where volatile fluid elements in the later stage of magma cooling encourage crystal formation. It also occurs in gneiss, micaschist, granulite, eclogite. Most gem sillimanite is found in gem gravels, carried by fast-flowing water from its headwater rocks to downstream sediment areas.
In the high-elevation Mogok region in Myanmar can be found violet-blue sillimanite stones, and in the Sri Lanka gem gravels, a grey-green chatoyant material is obtained. Group masses of greenish crystals have been found in Bohemia (Czechoslovakia); yellowish crystals in Val di Fassa (Trento, Italy), Bavaria and Saxony (Germany); specimens of chatoyant fibrolite aggregates in Brazil and South Carolina (USA). Blue crystals of sillimanite associated with rubies have been found in sands in Burma and Sri Lanka. Small-sized material similar to the Myanmar specimens is reported from Kenya. Large, remarkably pure masses (85% sillimanite) have been mined in India.
Sillimanite is also called bucholzite fibrolite - very rarely offered as a small gem quality stone but more often as a fibrous aggregate. Greenish or brownish varieties of sillimanite are used as an inferior substitute for jade. This has led to the unfortunate misleading term of 'false jade' to describe what is rightfully sillimanite.
How does Sillimanite differ from its polymorphs?
As one of three Al2SiO5 polymorphs, sillimanite forms at higher temperatures than andalusite but lower pressures than kyanite. Its orthorhombic structure creates distinct physical and optical properties.
What determines fibrous crystal development?
The characteristic fibrous habit results from structural chains of edge-sharing AlO6 octahedra. Growth conditions influence fiber development, affecting both size and orientation of crystals.
How do metamorphic conditions affect formation?
Formation occurs in high-grade metamorphic facies, typically above 550°C. Pressure-temperature conditions determine polymorph stability, making sillimanite an important geologic indicator mineral.
What causes color variations in specimens?
Pure sillimanite is colorless, but trace elements create various colors. Iron content produces brown to green hues, while other impurities can result in gray or pale blue coloration.
How are gem-quality specimens identified?
Gem-quality material requires high transparency and minimal inclusions. Strong pleochroism and distinct crystal orientation help distinguish natural specimens from similar-appearing minerals.
What analytical methods verify composition?
X-ray diffraction confirms crystal structure, while electron microprobe analysis determines exact chemical composition. Optical properties provide additional diagnostic criteria.
Why is locality significant for specimen value?
Classic localities like Mogok (Myanmar) and Orissa (India) produce specimens with superior transparency and crystal development. Regional geology influences crystal quality and collection value.
The specific gravity [?] for Sillimanite is 3.25, its refractive index [?] is 1.66-1.68, and its double refraction [?] is 0.019.
History
Sillimanite was first described in 1824 after it was found in Connecticut, (USA) and is named after the American chemist Benjamin Silliman, who was one of the first Yale University professors of science and and analyzed many minerals, identifying their constituent elements.
Industrial Usages
Sillimanite is an industrial mineral used in the manufacture of refractories for furnaces, ovens, incinerators, reactors; high-temperature crucibles, and high temperature-resistant ceramics; and glass manufacturing. Chatoyant, green and blue varieties are used in jewelry. Sillimanite is valuable to geological study as a sensitive indicator of the temperature and pressure at which the host rock formed.
It is cut as faceted gemstones and sometimes with blue-green chatoyant effect. Sillimanite is one of the most difficult transparent gem materials to fashion.
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