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Formula: (Ba,Ca,K)5(Si27Al9)O72.22H2O
Tectosilicate (framework silicate), heulandite subgroup,
zeolite group, barium-bearing
mineral
Crystal System: Monoclinic
Specific gravity: 2.35 measured, 2.35 calculated for the empirical formula
Hardness: 3½
Streak: White
Colour: Colourless to white, pale yellowish white, pale beige
Luminescence: Not fluorescent under UV
Solubility: Finely ground heulandite-Ba easily decomposes in warm 6M hydrochloric acid, leaving silica as a
powder. It is rather resistant to cold, dilute acid.
Environments
Heulandite-Ba is a late-stage mineral in hydrothermal
quartz-calcite veins. Associated
minerals include acanthite, baryte,
chalcopyrite, fluorite,
galena, sphalerite,
silver, brewsterite, other
heulandite subgroup
zeolites, calcite and
harmotome
(HOM).
Localities
The type locality, the Northern Ravnås Prospect, Vinoren Southern Mines, Vinoren Silver Mine Field, Flesberg,
Buskerud, Norway, is situated within a district that is dominated by strongly metamorphosed and tectonised rocks,
that are cut by dolerite dykes, hydrothermal
quartz veins and calcite veins
deposited along fissures and faults. It is suggested that the vein minerals were precipitated from hydrothermal
solutions at temperatures in the range 200 to 300oC at depths between 3 and 4 km, with a hydrostatic
pressure of about 355 bars.
Heulandite-Ba occurs in
calcite-quartz veins that range from
a few mm to 10 cm in thickness. Baryte,
pyrite, chalcopyrite,
sphalerite, galena,
silver, acanthite and
fluorite are also present. Late stage minerals include
zeolites of the
brewsterite subgroup, the
heulandite subgroup and
harmotome. A late generation
calcite usually filled the last empty space in the hydrothermal veins,
although small cavities are still quite frequently observed
(EJM 17.1.143-154).
Heulandite-Ba from the Northern Ravnås Prospect -
Image
At the Bratteskjerpet Mine, Kongsberg, Buskerud, Norway, heulandite-Ba with transition to
heulandite-Ca was identified in samples from the mine dumps. Also at
this locality, the minerals of the heulandite subgroup are
associated with minerals of the brewsterite subgroup,
harmotome, calcite and minor
pyrite
(EJM 17.1.143-154).
At Åmot, Sjoa, Sel, Innlandet, Norway, strongly zoned crystals composed of heulandite-Ba,
heulandite-Sr and
heulandite-Ca together with
quartz, hematite,
rutile, anatase,
chlorite, albite and minerals of
the brewsterite subgroup, were discovered in a sample from a
roadcut south of Åmot farm. The deposit represents a low-temperature, alpine vein mineralisation. The crystals are
strongly zoned, and often show oscillatory crystallisation. The inner part of the crystals is
strontian heulandite-Ca.
Discrete zones outwards are composed of calcian heulandite-Sr of
varied composition. The outer part of the crystals, with a relatively sharp boundary against the inner part, is
heulandite-Ba
(EJM 17.1.143-154).
Whitesmith Mine, Strontian, Fort William and Ardnamurchan, Highland, Scotland, UK.
In the description below, the mineral is simply referred to as "heulandite". Quantitative chemical analysis showed
that twelve of the 60 analyses fall within the heulandite-Ba composition field, the remaining seventy-two are
heulandite-Sr.
Heulandite, not previously known from Strontian, was first found at the Whitesmith Mine in 1979. The specimens
display blocky colourless crystals up to about 8 mm in length, with a slightly pearly lustre, in cavities with
hexagonal 'nailhead' calcite, cubo-octahedral
pyrite and occasional brewsterite.
The next interesting discovery was made in 2003. A careful examination revealed several parallel-running
baryte veins containing numerous cavities lined with drusy
brewsterite, acicular brown
strontianite and very rare heulandite. About ten
heulandite specimens were collected, and the finest of these contained a large cavity lined with acicular
brown strontianite and a scattering of colourless coffin-shaped
heulandite crystals to about 4 mm.
In 2008, a small collapse on the south wall of the opencut afforded an opportunity to examine another mineralised
area. Numerous thin calcite veins run parallel to the local cleavage and
where the rock is folded, larger cavities and fractures lined with
brewsterite and occasional weathered
strontianite are present. Heulandite was found as small
inconspicuous crystals in and around an otherwise unremarkable calcite vein
up to about 25 mm wide. The vein comprised two calcite cheeks between 5 and
10 mm in thickness, which commonly met, but sometimes opened to produce a narrow central cavity lined with
calcite, pyrite,
brewsterite and heulandite.
Where the vein intersected perpendicular veins and fractures, larger cavities lined with
brewsterite, calcite and
heulandite occasionally developed, the best of these contained heulandite crystals up to 18 mm long
(JRS 11.80-86).
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