Geographic Minerals

Boltwoodite: Properties and Occurrences

Boltwoodite: Properties and Occurrences

Boltwoodite is a hydrated potassium uranyl silicate mineral with formula HK(UO2)(SiO4)·1.5(H2O). It was named after Bertram Borden Boltwood (1870-1927), radiochemist of Yale University, New Haven, Connecticut, USA, who devised the U-Pb method of measuring geologic time. 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.

General Information

  • Category: Uranium silicate mineral
  • Formula; HK(UO2)(SiO4)·1.5(H2O)
  • Crystal system: Monoclinic
  • Crystal class: Prismatic (2/m) (same H-M symbol).


Boltwoodite can be identified in the field by its color variations between yellow and light yellow. Its opaque form has {010} perfect and {001} distinct cleavage. This mineral has a vitreous-pearly luster with a white streak. The density of boltwoodite is 3.6 g/cm3, with a hardness of 3.5-4 – between that of a copper penny and fluorite.

  • Color: Pale yellow, orange yellow
  • Crystal habit: Elongated crystals, acicular to fibrous
  • Cleavage: Perfect on {010}, imperfect on {001}
  • Tenacity: Brittle
  • Mohs scale hardness: 3.5 – 4
  • Luster: Silky to vitreous, dull or earthy in aggregates
  • Streak: White
  • Diaphaneity: Transparent to translucent
  • Specific gravity: 4.7


Boltwoodite occurs as secondary silicate alteration crusts surrounding uraninite and as fracture fillings. It occurs in the outer silicate zone of alteration surrounding hydrated uranyl oxides incrusting primary uraninite; and also filling fractures at some distance from primary uraninite. It is found in pegmatites and sandstone uranium deposits of the Colorado Plateau-type. It occurs associated with uraninite, becquerelite, fourmarierite, phosphuranylite, gypsum, and fluorite.

It also occurs in pegmatite and Colorado-Plateau-type sandstone uranium deposits. It is often associated with minerals such as uraninite, becquerelite, fourmarierite, gypsum, fluorite, and phosphuranylite.


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