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The heulandite group is a group of tectosilicates (framework silicates) and a sub-group of the
zeolite group, comprising:
heulandite-Ba:
(Ba,Ca,K)5(Si27Al9)O72.22H2O,
heulandite-Ca:
(Ca,Na,K)5(Si27Al9)O72.26H2O,
heulandite-K:
(K,Ca,Na)5(Si27Al9)O72.26H2O,
heulandite-Na:
(Na,Ca,K)5(Si27Al9)O72.22H2O,
heulandite-Sr:
(Sr,Ca,Na)5(Si27Al9)O72.24H2O
Crystal System: Monoclinic
Specific gravity: 2.2
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, yellowish, red
Solubility: Moderately soluble in hydrochloric acid
Environments:
Heulandite is usually found in cavities of mafic igneous rocks associated with
zeolites and calcite.
It also occurs as druses in pegmatites.
Localities
At the Marron Volcanics of the Olalla Area South-Central British Columbia, Canada, at the Yellow Lake road cuts red,
strontium-bearing heulandite crystals to 5 mm long have been found lining veins cutting
trachyte. The heulandite is commonly overgrown by
brewsterite, laumontite,
calcite, goethite and/or
fluorite.
At the White Lake Road occurrence, heulandite occurs as translucent to colourless crystals up to 1 cm in length,
commonly associated with large brewsterite crystals. Also a single nodule has
been found with heulandite interstitially grown in carbonate plates that were in turn covered in
thomsonite and natrolite
(R&M 96.6.522-523).
In Western Iceland geothermal wells have been drilled through a thick series of basalt
flows, where it was found that heulandite crystallised at temperatures from 65oC to 200oC at depths between
30m and 1200m
(ZW).
At Palabora, Limpopo Province, South Africa heulandite has been reported in association with
calcite, saponite, and
fluorapophyllite.
Heulandite is found associated with stilbite and
pyrite in water tunnels under Queens, New York City, USA (R&M 84.3.231),
and with stilbite, datolite and
chabazite at the Quarry Road Cut, also New York City (R&M 91.6.562-566).
In the Carlin trench, The Selleck Road Tremolite and Tourmaline Locality, West Pierrepont, St. Lawrence County, New York, USA,
heulandite occurs on weathering scapolite
(R&M 91.2.123).
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