Plášilite

Plášilite is a very rare uranium mineral with the formula Na₂(UO₂)(SO₄)₂•3H₂O.^[2][3] Chemically related minerals include natrozippeite, belakovskiite, meisserite, fermiite and oppenheimerite.^{[4][5][6][7][8]} Most of these uranyl sulfate minerals were originally found in the Blue Lizard mine, San Juan County, Utah, US.^[9] The mineral is named after Czech crystallographer Jakub Plášil.^[3]

Plášilite
General
Category	Sulfate mineral
Formula	Na(UO2)(SO4)(OH)•2H2O
IMA symbol	Pšl[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 8.71, b = 13.84, c = 7.05 [Å], β = 112.13° (approximated)
Identification
Color	Greenish yellow
Crystal habit	prismatic
Cleavage	{010} and {001}, perfect
Tenacity	Brittle
Mohs scale hardness	2-3
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Density	3.73 (calculated) (approximated)
Optical properties	Biaxal (+)
Refractive index	nα=1.56, nβ=1.58, nγ=1.61 (approximated)
Pleochroism	~Colourless (X), very pale yellow (Y), pale yellow (Z)
2V angle	88o (measured)
Ultraviolet fluorescence	Bluish-white
Other characteristics	Radioactive
References	[2][3]

Association and origin

Plášilite is associated with other sulfate minerals: natrozippeite, johannite, blödite, brochantite, chalcanthite, gypsum, hexahydrite, manganoblödite, and tamarugite. Non-sulfate coexisting minerals include atacamite, calcite, dickite and gerhardtite. Plášilite is secondary in origin, being the product of weathering of the primary uranium mineral, uraninite.^[2]

Crystal structure

The crystal structure of plášilite is of a new type. Its building blocks are:^[2]

• (UO₂)₂(SO₄)₂(OH)₂ sheets, with a charge 2-, parallel to (010), of a phosphuranylite topology
• edge-sharing NaO₂(H₂O)₄ polyhedra, parallel to [001]

The sodium-bearing polyhedra link the uranyl-sulfate sheets. It terms of sheet geometry, crystal structure of plášilite is similar to that of deliensite.^[2]