Heulandite

tectosilicate

basaltic cavities

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
Specific gravity: 2.2
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, yellowish, red
Solubility: Moderately soluble in hydrochloric acid
Environments:

Pegmatites
Basaltic cavities

Heulandite is usually found in cavities of mafic igneous rocks associated with zeolites and calcite. It also occurs as druses in pegmatites.
It 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).
It occurs on weathering scapolite in the Carlin trench, The Selleck Road Tremolite and Tourmaline Locality, West Pierrepont, St. Lawrence County, New York (R&M 91.2.123).
At Palabora, Limpopo Province, South Africa it has been reported in association with calcite, saponite, and fluorapophyllite.

Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that heulandite crystallised at temperatures from 65oC to 200oC at depths between 30m and 1200m (ZW).

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