Prehnite

prehnite
Prehnite

Often an oily green, prehnite may also range from pale yellowish to brown. Columnar or tabular crystals are rare; it occurs more usually as aggregates of barrel-shaped crystals or as botryoidal masses. Some pale yellowish brown prehnite is fibrous enough to be cut in cabochon, and may show the cat’s eye effect.

Prehnite is found in basaltic volcanic rocks, intrusive igneous rocks, and in some metamorphic rocks. Pale green masses are found in Scotland; dark green or greenish brown masses in Australia; aggregates of crystals in France.

Prehnite is named after Colonel von Prehn, who first introduced prehnite to Europe.

Cause of Color :

Iron

Chemical Composition :

Calcium Aluminium Silicate Ca2Al2Si3O10(OH)2

Crystal System / Forms :

Orthorhombic System / Aggregates with botryoidal habit.

Cuts & Uses :

Cabochons, beads, carvings, etc.

Hardness : 6
Lustre : Vitreous to greasy
Magnification :

Fibrous structure with radiating pattern, banding.

Optic Character :

Anisotropic, D.R.; Aggregate (A.G.G.)

Pleochroism : None.
Refractive Index / Birefringence :

1.615 – 1.650 / 0.035

Simulants (with separation tests) :

Jadeite (R.I., S.G., structure, spectrum), Serpentine (R.I., S.G., lustre, hardness), Idocrase (R.I., spectrum, structure), Hydrogrossular Garnet (R.I., S.G.), Aventurine Quartz (R.I., S.G., structure, spectrum), Maw-sit-sit (color, structure), Aragonite (R.I., birefringence, structure).

Sources :

U.S.A., France, Scotland, Australia, China.

Specific Tests :

Consist of fibers in radiating manner in different planes.

Spectrum :

Not Diagnostic.

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