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Formula: SeO2
Valence: Se4+O2
Simple oxide of selenium
Crystal system: Tetragonal
Specific gravity: 4.146 calculated
Streak: White
Colour: Colourless (red - included selenium, yellow - included sulphur)
Luminescence: No fluorescence under long wave or short wave UV
Solubility: Extremely hygroscopic. In air will deliquesce in a few minutes and be absorbed into substrate or evaporate
leaving a colourless film. Readily soluble in cold water, mineral acids, and acetone.
Environments
Localities
The type locality is the Forestville Coal Dump Fires, Schuylkill County, Pennsylvania, USA. Downeyite is a
sublimation product of escaping gases from vents in activately burning culm banks (spoil heaps) in the
anthracite region of
eastern Pennsylvania. It occurs as rare, transparent, acicular to prismatic crystals dispersed on a matrix of spent
shale, locally referred to as "red dog". Less commonly, the needle-like
crystals cluster to form radiating sprays. Crystals of downeyite were first observed on actively burning
anthracite culm banks near Forestville. The crystals are extremely
hygroscopic and unstable under normal atmospheric conditions, therefore, their presence and preservation on culm banks is
restricted to areas within the hot dry environment immediately surrounding active vents. Temperature measurements made in
the zones in which downeyite was forming were generally in the range of 190oC to 230oC.
Crystals of downeyite appear to have formed in association with
native selenium, rosickýite
(monoclinic sulphur), mascagnite and
godovikovite.
Crystals of downeyite are generally transparent and colourless, with an adamantine lustre. Some crystals, however,
appear red from bright red inclusions of amorphous selenium; others have a
yellowish tint at their base from a dusting of sulphur. Downeyite
commonly occurs in the form of fine acute needles, ranging from 0.1 mm to 2 cm in length.
The most striking feature of downeyite is its extremely hygroscopic character. Removed from the hot dry environment
in which they form, downeyite crystals quickly absorb water from the atmosphere and deliquesce in a few minutes,
forming droplets of a clear liquid which the substratum absorbs, or which evaporates, leaving an unidentified colourless
transparent film
(AM 62.316-320).
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