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Prehnite

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Formula:
Ca2Al2Si3O10(OH)2
System:
Orthorhombic
Colour:
Colourless to gray to ...
Lustre:
Vitreous, Pearly
Hardness:
6 - 6½
Name:
Named in 1788 by Abraham Gottlieb Werner in honor of the Dutch Colonel, Hendrik von/van Prehn [July 31/August 2, 1733 Cape of Good Hope Colony - August, 1785 Heilbronn, Wurttemberg {Germany}], who is credited with discovering the mineral in 1774 at the Cape of Good Hope in South Africa. Von Prehn was a governor of Cape of Good Hope (1779-1780) until he was dismissed. He was a naturalist and mineral collector who brought the the first specimens of his namesake mineral to Europe. It is unknown how the mineral came to the attention of Werner. A considerable mystique and folklore has surrounded Prehn and prehnite. The appelation "von" was supposedly bestowed on Prehn for his military service during the Seven Year's War, but there are no printed records available to substantiate the claim. It has been claimed that prehnite was the mineral named for a person. In 1813, Richard Chenevix wrote a lengthy article (100+ pages) decrying the naming of minerals for people.)

Classification of Prehnite

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
9.DP.20

9 : SILICATES (Germanates)
D : Inosilicates
P : Transitional ino-phyllosilicate structures
72.1.3.1

72 : PHYLLOSILICATES Two-Dimensional Infinite Sheets with Other Than Six-Membered Rings
1 : Two-Dimensional Infinite Sheets with Other Than Six-Membered Rings with 4-membered rings
16.9.10

16 : Silicates Containing Aluminum and other Metals
9 : Aluminosilicates of Ca
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Type Occurrence of Prehnite

Year of Discovery:
1788

Occurrences of Prehnite

Geological Setting:
A secondary mineral in mafic volcanic rocks, low grade metamorphic mineral.

Physical Properties of Prehnite

Vitreous, Pearly
Diaphaneity (Transparency):
Translucent
Comment:
Pearly on cleavage {001}
Colour:
Colourless to gray to yellow, yellow-green or white
Streak:
White
Hardness (Mohs):
6 - 6½
Tenacity:
Brittle
Cleavage:
Distinct/Good
Good on {001}, poor on {110}
Fracture:
Irregular/Uneven
Density:
2.8 - 2.95 g/cm3 (Measured)    2.9 g/cm3 (Calculated)

Crystallography of Prehnite

Crystal System:
Orthorhombic
Class (H-M):
mm2 - Pyramidal
Cell Parameters:
a = 4.646(2) Å, b = 5.491(3) Å, c = 18.52(3) Å
Ratio:
a:b:c = 0.846 : 1 : 3.373
Unit Cell Volume:
V 472.47 ų (Calculated from Unit Cell)
Z:
2
Morphology:
Uncommon crystals, aggregates are fanlike, reniform, globular, stalagtitic, granular, compact.
Twinning:
Fine lamellar
X-Ray Powder Diffraction:
Image Loading

Radiation - Copper Kα
Data Set:
Data courtesy of RRUFF project at University of Arizona, used with permission.
X-Ray Powder Diffraction Data:
d-spacingIntensity
3.48 (90)
3.28 (60)
3.08 (100)
2.55 (100)
2.37 (40)
2.31 (40)
1.77 (70)
(

Optical Data of Prehnite

Type:
Biaxial (+)
RI values:
nα = 1.611 - 1.632 nβ = 1.615 - 1.642 nγ = 1.632 - 1.665
2V:
Measured: 64° to 70°, Calculated: 58° to 68°
Max Birefringence:
δ = 0.021 - 0.033
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Moderate
Dispersion:
weak r > v

Chemical Properties of Prehnite

Formula:
Ca2Al2Si3O10(OH)2
Essential elements:
All elements listed in formula:
Common Impurities:
Ti,Fe,Mg,Na,K,H2O

Relationship of Prehnite to other Species

9.DP.05Meliphanite(Ca,Na)2(Be,Al)[Si2O6(OH,F)]
9.DP.15LeucospheniteBaNa4Ti2B2Si10O30
9.DP.25AmstalliteCaAl(Si,Al)4O8(OH)4 · (H2O,Cl)
9.DP.30KvanefjelditeNa4(Ca,Mn)(Si3O7)2(OH)2
9.DP.35Lemoynite(Na,K)2CaZr2Si10O26 · 5H2O
9.DP.35NatrolemoyniteNa4Zr2Si10O26•9H2O
9.DP.40AltisiteNa3K6Ti2Al2Si8O26Cl3
16.9.1GehleniteCa2Al(AlSiO7)
16.9.2AnorthiteCaAl2Si2O8
16.9.3Yoshiokaite(Ca,Na)[Al(Al,Si)O4]
16.9.4SvyatoslaviteCaAl2Si2O8
16.9.5DmisteinbergiteCaAl2Si2O8
16.9.6GrossularCa3Al2(SiO4)3
16.9.7VuagnatiteCaAl(SiO4)(OH)
16.9.8Zoisite{Ca2}{Al3}(Si2O7)(SiO4)O(OH)
16.9.9Clinozoisite{Ca2}{Al3}(Si2O7)(SiO4)O(OH)
16.9.11KamaishiliteCa2(Al2SiO6)(OH)2
16.9.12BicchuliteCa2(Al2SiO6)(OH)2
16.9.13MargariteCaAl2(Al2Si2O10)(OH)2
16.9.14ChantaliteCaAl2(SiO4)(OH)4
16.9.15CebolliteCa5Al2(SiO4)3(OH)4
16.9.17KatoiteCa3Al2(SiO4)3-x(OH)4x
16.9.18LawsoniteCaAl2(Si2O7)(OH)2 · H2O
16.9.19ParthéiteCaAl2Si2O8 · 2H2O
16.9.20WairakiteCa(Al2Si4O12) · 2H2O
16.9.21VertumniteCa4Al4Si4O6(OH)24 · 3H2O
16.9.22ScoleciteCaAl2Si3O10 · 3H2O
16.9.23LaumontiteCaAl2Si4O12 · 4H2O
16.9.24YugawaraliteCaAl2Si6O16 · 4H2O
16.9.25Gismondine-CaCaAl2Si2O8 · 4H2O
16.9.26EpistilbiteCaAl2Si6O16 · 5H2O
16.9.27GoosecreekiteCa[Al2Si6O16] · 5H2O
16.9.28CowlesiteCaAl2Si3O10 · 6H2O
16.9.30StelleriteCa4(Si28Al8)O72 · 28H2O
16.9.31SträtlingiteCa2Al2SiO7 · 8H2O
16.9.32AmstalliteCaAl(Si,Al)4O8(OH)4 · (H2O,Cl)

Other Names for Prehnite

Other Information

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Prehnite

Reference List:
F. Aumento (1968): The space group of prehnite. Can. Mineral. 9, 485-492.

European Journal of Mineralogy (1990): 2: 731-734.

Canadian Mineralogist (1987): 25: 707.

Internet Links for Prehnite

Specimens:
The following Prehnite specimens are currently listed for sale on minfind.com.

Localities for Prehnite

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
Mineral and/or Locality  
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