HEULANDITE

This article appears in Volume V13, Page 416 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: HEG-HIG

HEULANDITE , a

mineral

MINERAL

of the zeolite group, consisting of hydrous calcium and aluminium silicate, H4CaAl2(SiO3)s + 3H20. Small amounts of sodium and potassium are usually present replacing part of the calcium. Crystals are monoclinic, and have a characteristic coffin-shaped habit. They have a perfect cleavage parallel to the 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

MINERAL

is usually colourless or white, sometimes 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

appearance

APPEARANCE (from Lat. apparere, to appear)

, and differs from it chemically only
in containing rather less water of crystallization.
The two minerals may, however, be readily dis-
tinguished by the fact that in heulandite the
acute positive 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. Brooke arrived at the same result, giving
the name heulandite, after the mineral collector, HenryHeuland.
Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasion-ally in gneiss and metalliferous veins. The best specimens are from the basalts of Berufjord, near Djupivogr, in Iceland and the Faroe Islands, and the Deccan traps of the

Sahyadri

SAHYADRI

mountains near Bombay. Crystals of a brick-red colour are from Campsie Fells in

Stirlingshire

STIRLINGSHIRE

and the Fassathal in

Tirol

TIROL (or TYROL 1)

. A variety known as beaumontite occurs as small yellow crystals on syenitic schist near

Baltimore

BALTIMORE
BALTIMORE, GEORGE CALVERT, 1ST BARON (C. 1580—1632)

Maryland

MARYLAND

.
Isomorphous with heulandite is the strontium and barium zeolite brewsterite, named after Sir

David

Brewster

DAVID
_17811868_.html">BREWSTER, SIR DAVID (1781–1868)
BREWSTER, WILLIAM (c. 1566–1644)

. The greyish monoclinic crystals have the composition H4(Sr, Ba, Ca)Al2(SiOa)s+3H20, and are found in the basalt of the Giant's Causeway in Co. Antrim, and with harmotome in the lead mines at Strontian in

Argyllshire

ARGYLLSHIRE

. (L. J. S.)

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