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Ca2 Al2Si3O10(OH)2 A light-green to white mineral sorosilicate crystallizing in the orthorhombic system and generally found in reniform and stalactitic aggregates with crystalline surface; it has a vitreous luster, hardness is 6-6.5 on Mohs scale, and specific gravity is 2.8-2.9.



(named after the Dutch colonel Prehn, who discovered the mineral on the Cape of Good Hope), a sorosilicate mineral that has the chemical composition Ca2 Al[AlSi3O10]·(OH)2 with a trace of Fe replacing the Al. Prehnite crystallizes in the rhombic system. Clearly formed crystals usually have a prismatic, tabular habit and are rare. Prehnite is mainly found as nodular aggregates with radial-columnar structures. It is greenish yellow or gray. Its hardness on Mohs’ scale is 6.5 and its density is 2,800–2,950 kg/m3.

Prehnite is an ordinary hydrothermal mineral that occurs in the cavities of basic effusive rocks together with such minerals as zeolites, calcite, and epidote. It also forms in metamorphic rocks, for example, skarns, and during the decomposition of plagioclases in gabbro and diabases.

References in periodicals archive ?
Other secondary minerals are carbonate, sericite, prehnite, titanite, chlorite, and minor epidote and hematite.
La chlorite, la prehnite et la pumpellyite mineralisee de facon eparse presentes, indiquent que les roches ont atteint le stade d'un facies a prehnite-actinolite.
The sheet dykes are often crushed along different directions and are filled with Prehnite veins [15].
The left label shown here is from the Mineralogical Record Label Archive (from Ron Bentley's collection), and the one on the right (still attached to a prehnite specimen) was recently shown to us by Kelly Nash.
Although not a major producer of collector-quality specimens, it has yielded fine examples of heulandite, stilbite, calcite, chalcedony, datolite and prehnite.
The regular octahedrons rest on colorless and smoky quartz and are accompanied by other minerals including prehnite, sphene and adularia.
Beautiful yellow-green, transparent prehnite in spheres, hemispheres and "crests" on matrix of graphite-infused rock, from the Merelani mines, Arusha, Tanzania--see the photos in the recent "Tanzanite Issue" (September-October 2009)--has sold for rather high prices in some quarters during the past three years or so, but excellent toenails and small miniatures of the material could be had for $15 to $45 from The Crystal Circle, LLC (www.
Prehnite, calcite and sulfides were the main constituents found within the irregularly shaped vugs of this structure.
Around 2000 John Saul saw quite a few prehnite specimens from Merelani, but they were all badly battered.
The specimens came from a collecting site in the mine (now completely exhausted) discovered at some time between 1996 and 1999 by a young geologist from Thetford Mines, who originally classified the mineral as prehnite.
The mine at Asbestos produced wonderful specimens of orange grossular ("hessonite") garnets and some of the best prehnite, pectolite and vesuvianite specimens in the world.