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HEULANDITE

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Originally appearing in Volume V13, Page 416 of the 1911 Encyclopedia Britannica.
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HEULANDITE , a See also:

mineral of the zeolite See also:group, consisting of hydrous See also:calcium and See also:aluminium silicate, H4CaAl2(SiO3)s + 3H20. Small amounts of See also:sodium and See also:potassium are usually See also:present replacing See also:part of the calcium. Crystals are See also:monoclinic, and have a characteristic See also:coffin-shaped See also:habit. They have a perfect cleavage parallel to the See also:plane of symmetry (M in the figure), on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or See also:white, sometimes See also:brick-red, and varies from transparent to translucent. The hardness is 31-4, and the specific gravity 2.2. Heulandite closely resembles stilbite (q.v.) in See also:appearance, and differs from it chemically only in containing rather less See also:water of See also:crystallization. The two minerals may, however, be readily dis- tinguished by the fact that in heulandite the acute See also:positive See also:bisectrix of the optic axes emerges perpendicular to the cleavage. Heulandite was first separated from stilbite by A. Breithaupt in 1818, and named by him euzeolite (meaning beautiful zeolite); independ- ently, in 1822, H. J. See also:Brooke arrived at the same result, giving the name heulandite, after the mineral See also:collector, HenryHeuland.

Heulandite occurs with stilbite and other See also:

zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasion-ally in See also:gneiss and metalliferous See also:veins. The best specimens are from the basalts of Berufjord, near Djupivogr, in See also:Iceland and the Faroe Islands, and the See also:Deccan traps of the See also:Sahyadri mountains near Bombay. Crystals of a brick-red See also:colour are from Campsie Fells in See also:Stirlingshire and the Fassathal in See also:Tirol. A variety known as beaumontite occurs as small yellow crystals on syenitic schist near See also:Baltimore in See also:Maryland. Isomorphous with heulandite is the See also:strontium and See also:barium zeolite brewsterite, named after See also:Sir See also:David See also:Brewster. The greyish monoclinic crystals have the See also:composition See also:H4(Sr, Ba, Ca)Al2(SiOa)s+3H20, and are found in the See also:basalt of the See also:Giant's See also:Causeway in Co. See also:Antrim, and with See also:harmotome in the See also:lead mines at Strontian in See also:Argyllshire. (L. J.

End of Article: HEULANDITE

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