Sweetite

Sweetite has a general formula of Zn(OH)₂.^[2] The name is given after a curator of mineral department of the British Museum, Jessie May Sweet (1901–1979).^[6] It occurs in an oxidized vein in limestone bedrock with galena, ashoverite, wülfingite, anglesite, cerussite, hydrocerussite, litharge, fluorite, palygorskite and calcite.^[5]

Sweetite
General
Category	Hydroxide mineral
Formula	Zn(OH)2
IMA symbol	Sw[1]
Strunz classification	4.FA.10
Crystal system	Tetragonal Unknown space group
Crystal class	Trapezohedral (422) H-M symbol: (422)
Space group	P41212 (no. 92) or P43212 (no. 96)
Unit cell	a = 8.22, c = 14.34 [Å] V = 968.93 Å3; Z = 20
Identification
Formula mass	99.40 g/mol
Color	Colorless, white
Crystal habit	Bipyramidal
Cleavage	None
Fracture	Irregular
Mohs scale hardness	3
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.33
Optical properties	Uniaxial (−)
Refractive index	nω = 1.635 nε = 1.628
Birefringence	0.007
References	[2][3][4][5]

Sweetite is tetragonal, which means crystallographically it contains one axis of unequal length and two axes of equal length. The angles between three of the axes are all 90°. It belongs to the space group 4/m. Some crystals show evidence of a basal plane and a few are tabular.^[7] In terms of its optical properties, sweetite has two indices of refraction, 1.635 along the ordinary ray and 1.628 along the extraordinary ray.^[8] The index of refraction is the velocity of light in vacuum divided by the velocity of light in medium. It also has the birefringence of 0.007.^[9] The birefringence means the decomposition of light into two rays when passing through a mineral. Sweetite is 1.64–1.65 in relief, which is medium to high in intensity and means a measure of the relative difference between the index of refraction of a mineral and its surrounding medium.^[2]

Sweetite is mostly found from a limestone quarry 200–300 m northwest of Milltown, near Ashover, Derbyshire, England.^[5]