prehnite

Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system. It is brittle with an uneven fracture and a vitreous to pearly lustre. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its color varies from light green to grey or white. It is translucent.
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(n.)
A pale green mineral occurring in crystalline aggregates having a botryoidal or mammillary structure, and rarely in distinct crystals. It is a hydrous silicate of alumina and lime.
  

General Information:
 Chemical Formula:
Ca2Al2Si3O10(OH)2
 Composition:
(Molecular Weight = 395.38 gm)
Calcium 20.27 % Ca
Aluminum 13.65 % Al
Silicon 21.31 % Si
Hydrogen 0.25 % H 
Oxygen 44.51 % O 
 Empirical Formula:
Ca2Al2Si3O10(OH)
 Environment:
Hydrothermally altered rocks.
 Locality:
Haslach, Harzburg and Oberstein, Germany.
 Name Origin:
Named after the Dutch Colonel, H. Von Prehn (1733-1785).
Physical Properties:
 Cleavage:
[001] Distinct
 Color:
colorless, gray, yellow, yellow green, or white.
 Density:
2.8 - 2.95, Average = 2.87
 Diaphaniety:
Subtransparent to translucent
 Habits:
Reniform - "Kidney like" in shape (e.g.. hematite)., Globular - Spherical, or nearly so, rounded forms (e.g. wavellite)., Stalactitic - Shaped like pendant columns as stalactites or s talagmites (e.g. calcite).
 Hardness:
6-6.5 - Orthoclase-Pyrite
 Luminescence:
Non-fluorescent.
 Luster:
Vitreous - Pearly
 Streak:
colorless
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