- Heulandite
Heulandite is the name of a series of tecto-silicate
mineral s of thezeolite group. Prior to 1997, heulandite was recognized as a mineral species, but a reclassification in 1997 by theInternational Mineralogical Association changed it to a series name, with the mineral species being named heulandite-Ca, heulandite-Na, heulandite-K, and heulandite-Sr. Heulandite-Ca, the most common of these, is a hydrous hydrous calcium andaluminium silicate , (Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O. Small amounts ofsodium andpotassium are usually present replacing part of the calcium.Strontium replaces calcium in the heulandite-Sr variety. The appropriate species name depends on the dominant element. The species are visually indistinguishable, and the series name heulandite is still used whenever testing has not been performed.Crystals are
monoclinic . They may have a characteristic coffin-shaped habit, but may also form simple rhombic prisms. Frequently, a crust of fine crystals will form with only the ends of the rhombs visible, making the crystals look like wedges. They have a perfect cleavage parallel to the plane of symmetry, on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or white, but may be orange, brown, yellow, brick-red, or green due toinclusion s ofceladonite . It varies from transparent to translucent.Isomorphous with heulandite is thestrontium andbarium zeolitebrewsterite .The Mohs' hardness is 3-4, and the specific gravity 2.2. Heulandite is similar to
stilbite . The two minerals may, however, be readily distinguished by the fact that in heulandite the acute positivebisectrix of the optic axes emerges perpendicular to the cleavage.Heulandite was first separated from stilbite by
August Breithaupt in 1818, and named by him "euzeolite" (meaning beautiful zeolite); independently, in 1822,H. J. Brooke arrived at the same result, giving the name heulandite, after the mineral collector,Henry Heuland (1778-1856).Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of
basalt icvolcanic rock s, and occasionally ingneiss and hydrothermal veins.The best specimens are from the basalts of
Berufjord , nearDjupivogr , inIceland and theFaroe Islands , and theDeccan traps of theSahyadri Mountains nearBombay . Crystals of a brick-red colour are fromCampsie Fells inStirlingshire and theFassathal inTirol . A variety known as beaumontite occurs as small yellow crystals on syeniticschist nearBaltimore inMaryland .References
* [http://www.mindat.org/min-6988.html Mindat Heulandite-Ca]
* [http://webmineral.com/data/Heulandite-Ca.shtml Webmineral Heulandite-Ca]
* [http://mineral.galleries.com/minerals/silicate/heulandi/heulandi.htm Mineral Galleries]
* [http://www.minsocam.org/MSA/IMA/ima98(13).pdf IMA Zeolite Classification]
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