Naumannite
A variety of Minerals
What is Naumannite?
Silver selenide (Ag2Se) is the reaction product formed when selenium toning analog silver gelatine photo papers in photographic print toning. The selenium toner contains sodium selenite (Na2SeO3) as one of its active ingredients, which is the source of the selenide (Se) anion combining with the silver in the toning process. It is found in nature as the mineral naumannite, a comparatively rare silver mineral which has nevertheless become recognized as important silver compound in some low-sulfur silver ores from mines in Nevada and Idaho.
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Key Characteristics
Composition of Naumannite
Silver selenide has two crystal phases. At lower temperatures, it has an orthorhombic structure, β-Ag2Se. This orthorhombic phase, stable at room temperature, is a narrow-gap semiconductor, with space group P212121. The exact size of the band gap has been given variously from 0.02 eV to 0.22 eV. There is also a high temperature cubic phase, α-Ag2Se., which it transforms into at temperatures above 130 °C. This high temperature phase has space group Im3m, No. 229, Pearson symbol cI20. The phase transition increases ionic conductivity by 10,000 times to about 2 S/cm.
Quick Facts
Physical Properties
- Color
- grey-black
- Hardness (Mohs)
- 2.5
- Density
- 8.24 g/cm³
- Streak
- black
Chemical Properties
- Chemical Formula
- Ag2Se
- Elements
- Ag, Se
Also Known As
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