Nesosilicate (insular SiO4 groups), uranophane group
Specific gravity: 4.7
Hardness: 3½ to 4
Streak: White
Colour: Pale yellow, orange yellow
Luminescence: Dull green in both short and long wave UV
Common impurities: Fe,Ca,Na

Plutonic igneous environments
Sedimentary environments

Boltwoodite typically occurs as clusters of radiating acicular to fibrous crystals with individual crystals to 2cm, but usually smaller. Also as poorly crystallised earthy masses at some localities (R&M 96.3.238-245). Boltwoodite is found in the outer silicate zone of alteration surrounding hydrated uranyl oxides incrusting primary uraninite; also filling fractures at some distance from primary uraninite. From pegmatite and Colorado-Plateau-type sandstone uranium deposits. Associated minerals include uraninite, becquerelite, fourmarierite, phosphuranylite, gypsum and fluorite (HOM).


At Arandis, Arandis Constituency, Erongo Region, Namibia, boltwoodite occurs in radial aggregates up to 2 cm long (Dana).
br> At the Goanikontes, Goanikontes, Arandis Constituency, Erongo Region, Namibia, spectacular specimens of bright orange sprays of boltwoodite to 7 mm occur on light coloured and dark calcite. The boltwoodite is found in light-coloured granite that hosts both primary and secondary uranium mineralisation. It consists of bright yellow to orange prismatic crystals to 2 cm that fluoresce reddish orange in long wave UV, but show no response under short wave. Most of the specimens are on a matrix of masses of small scalenohedral crystals of calcite, frequently coated by dark iron or manganese oxides. These oxides include cryptomelane, goethite, ranciéite, pyrolusite and uraninite, and they are also found as inclusions in the calcite. Later overgrowths of gypsum on the boltwoodite are common. (R&M 96.3.238-245).

At the New Method Mine, Amboy area, Bristol Mountains, San Bernardino county, California, USA, small free-standing sprays of lemon-yellow crystals of boltwoodite occur, some of which are associated with small calcite crystals and perfect cubes of dark purple mm sized fluorite (R&M 96.3.238-245).

At the Sherrer Quarry, C.K. Williams Quarry complex, Chestnut Hill, Easton, Northampton county, Pennsylvania, USA, boltwoodite occurs as an alteration of thorian uraninite (Dana).
br> At the type locality, Pick's Mine, Delta, San Rafael Mining District, Emery county, Utah, USA, boltwoodite occurs as wart-like aggregates of fibres coating fractures in sandstone. It is an oxidation product of primary black uranium ores (AM 42.307-308). Associated minerals include becquerelite, brochantite, fluorite, fourmarierite, gypsum, phosphuranylite, uraninite, pyrite, tyuyamunite and zippeite (R&M 96.3.238-245).

Back to Minerals