Photo of Dravite in Processed & Rough Form
Dravite is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of complex borosilicate, their full chemical compound being NaMg3Al6(BO3)Si6O18(OH)4.
Dravite also called Brown Tourmaline is a sodium magnesium-rich mineral in the Tourmaline group of silicates, with a hexagonal system. Other members of this group include elbaite (sodium lithium aluminum rich); schorl and buergerite (sodium iron rich); uvite (calcium magnesium rich); and liddicoatite (calcium lithium aluminum rich). A continuous solid solution series exists between uvite and dravite (calcium-sodium substitution in sodium- and magnesium-rich tourmalines respectively).
Crystals in the tourmaline group are prismatic, often very elongated with vertical striations and sometimes hemimorphic. There are aggregates of parallel or radiating individuals. Stubby prismatic crystals are less frequent. Compact masses are rare.
Dravite occurs as little radiating tuffs along twinning planes in albite or in albite crystal as isolated needles. Equant forms do not usually show striations. Crystals may show a triangular cross-section with curved sides. Inclusions may be so placed that a cat's-eye effect is possible though the eye is less sharp than its counterpart in chrysoberyl.
It is very hard, heavy, with conchoidal fracture. It is transparent to translucent with vitreous luster, tending to resinous on fracture surfaces. The colour is mainly yellow brown to dark brown, but may be brown to black, red, yellow, blue or green, colourless to white. It is insoluble in acids.
It also appears as lathlike inclusions, as intergrowth of golden-brown or yellow dravite with pyrrhotite and chalcopyrite, and rarely with galena, sphalerite, or pyrite, from Black Hawk, Penobscot Bay area, Mine (USA). Poikilitic aligned laths, an intergrowth of plagioclase subparallel to the c-axis of dravite can be seen in Yinnieharra district (West Australia).
A fibrous flexible matted aggregate of dravite parallel to c-axis can be in Rutherford, Virginia (USA). Dark brown to red dravites come from Zambia (Africa).
A green to black magnesium and chromium-rich variety is called "chromdravite" which occurs as small acicular dark green crystals and has been found in Kareliya, Russia; a dark brown to black magnesium-rich variety is "ferridravite" or "povondravite," very small black crystals found in deposits in Bolivia.
Terms used for dravite include "magnesium tourmaline" a scientific designation; "coronet," for dravite from Crown Point, New York (USA); "magnesioschorlite" for the dravite part of a dravite-schorl series; "mangano dravite" for mangan-rich dravite tourmaline; "vanadiumdravite," very small dark green to yellow crystals at locations in the Lake Baikal area of Russia.
What distinguishes Dravite from other tourmaline species?
Dravite represents the magnesium-rich end member of the tourmaline group, with chemical formula NaMg3Al6(BO3)3Si6O18(OH)4. Its distinctive composition affects optical and physical properties, typically producing brown to yellow-brown coloration.
How does chemical composition affect color variation?
Color zoning in dravite results from variations in transition metal content, primarily iron and titanium. Chromium substitution can produce green varieties, while vanadium content may create emerald-green specimens.
What are the diagnostic properties for identification?
Key identifying characteristics include trigonal crystal system, distinctive triangular cross-section, hardness of 7-7.5, and strong pleochroism. Refractive indices and specific gravity vary with composition.
How do formation conditions affect crystal development?
Dravite typically forms in metamorphic environments rich in magnesium, particularly in marble and dolomitic limestones. Crystal size and quality correlate with formation temperature and pressure conditions.
What causes sector zoning in Dravite crystals?
Sector zoning results from differential incorporation of elements during crystal growth along different crystallographic faces. This creates distinctive color patterns that can aid in crystallographic orientation determination.
How is chromian Dravite distinguished analytically?
Chromian dravite requires specific analytical techniques including UV-Vis-NIR spectroscopy and electron microprobe analysis to confirm chromium content and distribution within the crystal structure.
What explains the piezoelectric properties?
The polar nature of dravite's crystal structure results in piezoelectric and pyroelectric properties. These characteristics derive from the asymmetric arrangement of ions in the crystal lattice.
The specific gravity [?] for Dravite is 3.06, its refractive index [?] is 1.61-1.63, and its double refraction [?] is 0.018.
History
Dravite derives its name from Drave in Austria.
Industrial Usages
Dravite is also used for ornamental applications, and as a gemstone. Because of its strong pleochroism, the stone must be cut so that the table lies parallel to the main axis. Varieties that are strongly piezoelectric and pyroelectric are used in high pressure gauges. The chemical complexity of minerals of the tourmaline group has so far ruled out serious attempts at synthesis.
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