Prehnite
Current inventory:  1 gem
 

  
Prehnite is named after Dutch Colonel, Hendrik von Prehn (1733 - 1785) who first found the material on the Cape of Good Hope.

Discovered in 1788;   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:

1788

View mineral photos:

Prehnite Mineral Photos and Locations

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 gems for sale: