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Formula: Zn4(SO4)(OH)6.5H2O
Hydrated sulphate of zinc,
namuwite group
Crystal System: Triclinic
Specific gravity: 2.70 measured, 2.75 calculated for the empirical formula
Hardness: 1
Streak: White to pale blue
Colour: Pale blue to colourless or white
Luminescence: No fluorescence under UV
Solubility: Easily soluble in dilute hydrochloric and nitric acids
Environments
Partial dehydration of osakaite may result in lahnsteinite
(Mingen).
Localities
The type locality, the Hirao mine, Onsen-cho, Minoh City, Osaka Prefecture, Japan, is developed in
shale. The primary
ore-minerals are sphalerite,
chalcopyrite, pyrite and
galena. Among them, sphalerite is
predominant, and the mineral is disseminated through the host rock, which consists of
chlorite altered during a hydrothermal alteration process. In the mine, many
secondary minerals such as
smithsonite, hydrozincite,
schulenbergite, the zinc-dominant
analogue of schulenbergite,
brianyoungite,
hemimorphite, brochantite,
ramsbeckite, aurichalcite,
malachite, azurite,
posnjakite and ktenasite occur
on the gallery wall and in cracks in the altered shale.
Osakaite is one of the secondary minerals; it occurs in
two different aspects. The first type occurs as hexagonal platy crystals up to 0.5 mm across and 0.01 mm in thickness
in the form of stalactitic aggregates up to 2 cm long. The second type occurs in cracks in the altered
shale. The associated minerals are
hydrozincite, smithsonite,
chlorite and limonite.
It is likely the osakaite was formed in zinc-,
copper- and SO4- bearing groundwater derived from
sphalerite and chalcopyrite
in the host rock at around 20°C
(CM 45.6.1511-1517).
Osakaite from the Hirao Mine - Image
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