| Boltwoodite | |
| Category: | Uranium silicate mineral |
| Formula: | (K0.56Na0.42)[(UO<sub>2</sub>)(SiO<sub>3</sub>OH)]·1.5(H2O) |
| Imasymbol: | Bdw[1] |
| Strunz: | 9.AK.15 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | P21/m |
| Unit Cell: | a = 7.0772(8) Å, b = 7.0597(8) Å, c = 6.6479(7) Å; β = 104.982(2)°; Z = 2 |
| Color: | Pale yellow, orange yellow |
| Habit: | Elongated crystals, acicular to fibrous |
| Cleavage: | Perfect on, imperfect on |
| Tenacity: | Brittle |
| Mohs: | 3.5–4 |
| Luster: | Silky to vitreous, dull or earthy in aggregates |
| Streak: | White |
| Diaphaneity: | Transparent to translucent |
| Gravity: | 4.7 |
| Opticalprop: | Biaxial (−) |
| Refractive: | nα = 1.668 – 1.670 nβ = 1.695 – 1.696 nγ = 1.698 – 1.703 |
| Birefringence: | δ = 0.030 – 0.033 |
| Pleochroism: | Weak, X = colorless, Y = Z = yellow |
| Fluorescence: | Fluoresces dull green in both SW and LW UV |
| Other: | Radioactive |
| References: | [2] [3] [4] [5] [6] |
Boltwoodite is a hydrated uranyl silicate mineral with formula (K0.56Na0.42)[(UO<sub>2</sub>)(SiO<sub>3</sub>OH)]·1.5(H2O), distinct in crystal structure from sodium boltwoodite, which has an orthorhombic structure rather than monoclinic.[7] It is formed from the oxidation and alteration of primary uranium ores. It takes the form of a crust on some sandstones that bear uranium. These crusts tend to be yellowish with a silky or vitreous luster.[5] [8]
It was first described in 1956 for an occurrence in Pick's Delta Mine, Delta, San Rafael District (San Rafael Swell), Emery County, Utah, US.[4] It is named after Bertram Boltwood (1870–1927) an American pioneer of radiochemistry.
Boltwoodite occurs as secondary silicate alteration crusts surrounding uraninite and as fracture fillings. It is found in pegmatites and sandstone uranium deposits of the Colorado Plateau-type. It occurs associated with uraninite, becquerelite, fourmarierite, phosphouranylite, gypsum and fluorite.[3]