ClassicGems.net

Prehnite

Discovered in 1774; IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

Ca₂Al₂Si₃O₁₀(OH)₂

Calcium Aluminum Silicate Hydroxide

Molecular Weight:

395.38 gm

Composition:

Calcium

20.27 %

Ca

28.37 %

CaO

Aluminum

13.65 %

Al

25.79 %

Al₂O₃

Silicon

21.31 %

Si

45.59 %

SiO₂

Hydrogen

0.25 %

H

2.28 %

H₂O

Oxygen

44.51 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/G.07-30

Nickel-Strunz 10th Ed. ID:

9.DP.20

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

Related to:

None

Members of Group:

None

Varieties:

None

Synonyms:

Adelite (of de Fourestier), Adilite, Aedelite, Ædelite (of Walmstedt), Aedilite, Bostrichites, Chiltonite, Chrysolite (of Sage), Coupholite, Prehnitoid (of Bechi)

Crystal Data

Crystallography:

Orthorhombic - Pyramidal

Crystal Habit:

Uncommon as crystals, tabular or prismatic to steep pyramidal, to 4.5 cm. Aggregates are fanlike, reniform, globular, stalactitic; granular, compact.

Twinning:

Fine lamellar

Physical Properties

Cleavage:

Good on [001], Poor on [110]

Fracture:

Irregular/Uneven

Tenacity:

Brittle

Moh's Hardness:

6.0 - 6.5

Density:

2.80 - 2.95 (g/cm³)

Luminescence:

Fluorescent; blue white or mild peach under SW UV, yellow under LW UV

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, light to dark Green, Yellow, White, Gray

Transparency:

Semitransparent to Translucent

Luster:

Vitreous to Pearly; Pearly on cleavage {001}

Refractive Index:

1.611 - 1.665  Biaxial ( + )

Birefringence:

0.0220 - 0.0330

Dispersion:

Weak; r > v

Pleochroism:

None

Occurances

Geological Setting:

A secondary or hydrothermal mineral in veins and cavities in mafic volcanic rocks; less commonly in granite gneiss or syenite; a typical product of low-grade metamorphism.

Common Associations:

Zeolites, Datolite, Pectolite, Calcite, Epidote, Copper

Common Impurities:

Ti, Fe, Mg, Na, K, H₂O

Type Locality:

Karoo dolerites, Craddock District, Eastern Cape Province, South Africa

Year Discovered:

Discovered in 1774; named in 1788

View mineral photos:

Prehnite Mineral Photos and Locations

Unusual Gem Categories

Fluorescent Gems

More Information

Mindat.org
Webmineral.com

Prehnite is a beautiful gem with a pearly to velvety look that glows with color in well faceted gems. Most facetable material is found in Australia and Mali. The richest, most saturated colors come from Australia. Colors range from bright yellow (Australia), yellowish green, pale green (Mali) and colorless.

Prehnite was named in 1788 by German geologist Abraham Gottlieb Werner (1749-1817) in honor of Dutch Colonel Hendrik von Prehn (1733-1785), who is credited with discovering the mineral in 1774 at the Cape of Good Hope in South Africa. Von Prehn was governor of Cape of Good Hope from 1779 to 1780. It has been claimed that Prehnite was the first mineral named for a person.

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