Mineral - Prehnite
Prehnite
Chemical Properties
Mineral Class
Silicates
Toxicity
none
Formula
Ca₂Al[(OH)₂|AlSi₃O₁₀]
Unit Cell
a=4.65 b=5.49 c=18.52 Z=2
Physical Properties
Hardness
6 - 6.5
Streak
white
Density
Normal (2-3,5)
Cleavage
Distinct after {001}, indistinct after {110}
Fracture
uneven
Tenacity
Brittle
Optical Properties
Color
redyellowgreencolorless
Birefringence
0.033
Luster
Vitreous
Transparency
translucent,
transparent
Pleochroism
Geomineralogical Properties
Crystal System
orthorhombic
Point Group
orthorhombic-pyramidal - mm2
Space Group
Pma2 (28)
Habit
spherical, prismatic, tabular, grape-shaped
Formation
(1) Hydrothermally, through the alteration of calcium-rich plagioclases in basic igneous rocks (e.g., basalt and gabbro) under low-grade metamorphic conditions of the zeolite to prehnite-pumpellyite facies.
(2) As a fracture-filling mineral formed by the circulation of hydrothermal fluids within igneous or metamorphic rock bodies.
Paragenesis
Twinning
Rarity
Common
Synonyms
Prehnite
Composition
Elements
Mass
SiO₂
41.67
TiO₂
0.12
Al₂O₃
24.44
Fe₂O₃
1.03
FeO
0.32
MgO
0.25
CaO
27.25
Na₂O
0.18
K₂O
0.18
H₂O
4.44
Canada (Dana 1895)