The heulandite group is a group of tectosilicates (framework silicates) and a sub-group of the zeolite group, comprising:
Specific gravity: 2.2
Hardness: 3½ to 4
Colour: Colourless, white, yellowish, red
Solubility: Moderately soluble in hydrochloric acid
Heulandite is usually found in cavities of mafic igneous rocks associated with
zeolites and calcite.
It also occurs as druses in pegmatites.
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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