Amstallite

Formula:  CaAl(Si,Al)4O8(OH)4.(H2O,Cl)

Origin of name:  From type locality; Amstall, Austria

 

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:

Unit cell: a = 18.830 Å, b = 11.517 Å, c = 5.190 Å, b = 100.86°

 

Colour:  Colourless

Diaphaneity: Transparent to translucent

Luster:  Vitreous

Habit:  Prismatic to acicular to 10 mm long x 0.5 mm across, rhombic to hexagonal cross section, commonly striated parallel [001]

Hardness:  3-5

Specific gravity:  2.4

Cleavage:  1; {100} good

Tenacity: Brittle

Streak: White

 

Synonyms/varieties:

 

Comments: Found in open fissures cutting pegmatite schlieren, in hydrothermally altered graphite-bearing metamorphic rocks

 

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