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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 (repeating unit) | 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]
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