Mineral description : Prismatine

Prismatine from Stakholmen in enstatite, talc and sillimanite.
Another piece of prismatine from Stakholmen. Partwise transparent.

Mineral class Struntz VIII/B.31 - 12. Neso - subsilicate
Chemical formula (Mg,Al,Fe2+)5-6Al4(0,0H,F)3BO3(Si,Al)04
Crystal system Orthorombic.
Habitus Crystals shortprismatic. Also massive and fibric.
Cleavage [110] indistinct. Fracture brittle, uneven.
Hardness 7 (kornerupine)
Density 3.3-3.34 (kornerupine)
Color Brownish yellow to brownish green, greenish brown, grayish, white, blue and colorless.
Streak White
Luster Vitreous.
Description  
Occurance Created by methamorphism of boron rich clay sediments.
In the Reading Prong locality the peak metamorphism occured at 670-740 degrees celsius and at a pressute about 6.2-8 Kbar.
Associates Biotite, sillimanite, oligoclase, garnet, tourmaline.
Litterature
DAVIS A. YOUNG
KORNERUPINE-GROUP MINERALS IN GRENVILLE GRANULITE-FACIES PARAGNEISS, READING PRONG, NEW JERSEY
Department of Geology, Geography, and Environmental Studies, Calvin College, Grand Rapids, Michigan 49546, U.S.A.
Nytt mineral för Sverige.
Litofilen nr 1,1992 (Västerås amatörgeologiska sällskap)
BGS Bulletinen Nr 1 - 1997 (Bergslagens Geologiska Sällskap)
Notes A member of the kornerupine group of minerals. It was long considered to be a boron rich variety of kornerupine, but recent years analysis showed it to be so special it finally in 1996 was considered to be an unique mineral.
The type locality of this mineral is in Waldheim, Sachsen, Germany, where it was found in 1886.
The Swedish sample of kornerupine in the museum of natural history were reexamined in 1996/97 ??? and found to consist of enough boron to be classed as a prismatine.
Locations A rare mineral only known from a few localities in the world.
  • Stakholmen, Hassela, Hälsingland.
    As gray crystal groups with enstatite, sillimanite and chondrodite.
  • Reading Prong, New Yersey, USA. Crystals up to 6 cm in length.
    Typically altered to muscovite rich mixtures.
  • Waldheim, Sachsen, Germany. Together with oligoclase, sillimanite, garnet and tourmaline.

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Last changed : 1998-03-29