All natural untreated Prehnite rough stone from Namibia.

Weight: 128 gram

Size: 65x53x32 mm

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Prehnite

Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system. It is brittle with an uneven fracture and a vitreous to pearly lustre. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its color varies from light green to blue or white. It is translucent.

Though not a zeolite, it is found associated with them and with datolite, calcite, etc. in veins and cavities of basaltic rocks, sometimes in granites, syenites, or gneisses. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies. It was first discovered in South Africa by Colonel Hendrik Von Prehn, an early Dutch governor of the Cape of Good Hope colony. Prehnite was the first mineral to be named after a person.

Prehnite was named after its discoverer; Colonel Hendrik von Prehn and is an attractive collection mineral that is occassionally used for ornamental stone purposes. Its color is usually a pleasant green and is at times quite unique to prehnite. Typical prehnite forms rather thick crusts with a rough or crystaline texture. Epimorphs (crystal growth over the surface of another mineral) over laumonite crystals are interesting and attractive. Usually the laumontite has dissolved away leaving the hollow crust of prehnite behind.

PHYSICAL CHARACTERISTICS:

Color is usually a pale green to a yellowish grass green, also gray, white or colorless.

Luster is vitreous to waxy or pearly.

Transparency: Crystals are transparent to mostly translucent.

Crystal System is orthorhombic; 2 m m.

Crystal Habits include nodular, concretionary, radial, encrusting and stalactitic formations among other similar types. Tabular or pyramidal individual crystals are rare but some nodular specimens show tabular crystal protrusions. Epimorphs (crystal growth over the surface of another mineral) over laumontite are usual, but available (see above).

Cleavage is good in one direction (pinacoidal).

Fracture is uneven.

Hardness is 6 - 6.5.

Specific Gravity is approximately 2.9+ (average).

Streak is white.

Other Characteristics: Lacks the luster of smithsonite and cleavage surfaces are curved and pearly.

Associated Minerals include datolite, gyrolite, fluorapophyllite, quartz, calcite, copper, pectolite, stilbite and other zeolites.

Notable Occurrences: South Africa, USA, India, Germany, Austria, Scotland, Namibia, Canada; China, Australia and France.

Best Field Indicators are crystal habit, color, cleavage, hardness and associations.