Chemistry:Kasolite

Short description: Lead uranyl silicate monohydrate mineral

Kasolite
Kasolite and malachite from Musonoi mine, Democratic Republic of Congo
General
Category	Mineral
Formula (repeating unit)	Pb(UO2)(SiO4) · H2O
Strunz classification	09.AK.15
Dana classification	53.03.01.01
Crystal system	Monoclinic
Crystal class	Prismatic H-M symbol: 2/m
Space group	P 21/a
Unit cell	596.48
Identification
Formula mass	587.33
Color	Reddish orange, yellow, yellow brown, green, gray green
Cleavage	Perfect on {001} Good on {010}, {100}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	4 - 5
|re|er}}	Resinous, Greasy
Streak	Light brownish yellow
Diaphaneity	Transparent, translucent, opaque
Specific gravity	5.83 - 6.5
Density	Measured: 5.83 - 6.5 Calculated: 6.256
Optical properties	Biaxial (+)
Refractive index	nα = 1.890 nβ = 1.910 nγ = 1.950
Birefringence	0.060
Pleochroism	Weak X =Y = Pale yellow Z = Colorless to slightly grayish
2V angle	Measured: 43° Calculated: 72°
Dispersion	Strong
Ultraviolet fluorescence	Radioactive
Common impurities	As, P, Ba, Fe, Mg, Ca
Other characteristics	Radioactive

Kasolite is an uncommon lead uranyl silicate monohydrate mineral. It is an IMA approved mineral, that had been a valid species before the foundation of the association, that had been first described and published in 1921 by Schoep. It is a grandfathered mineral, meaning the name kasolite is still believed to refer to a valid species to this day. The mineral's name originates from its type locality, namely the Shinkolobwe Mine, also known as Kasolo Mine. Kasolite is possibly the lead analogue of the unnamed phase UM1956-02-SiO:CaHU, and it is the only accepted lead-uranium silicate.

Visual properties

Kasolite occurs in prismatic, lath-like crystals. It typically forms either clusters of radial acicular crystals, microcrystals, or a coating on top of the altered specimens. Individual crystals can grow up to a few millimeters. Massive specimens tend to have a dull to earthy luster. Kasolite is a weakly pleochroic mineral, which is an optical phenomenon. The mineral's color seems to be changing depending on the axis it is inspected on. On the X and Y axes, it appears in a pale yellow color, while on the Z, it can have a colorless to slightly grayish coloration.

Chemical properties

Kasolite mainly consists of uranium (40.53%), lead (35.28%) and oxygen (19.07%), but otherwise contains silicon (4.78%) and a small amount of hydrogen (0.34%). It has a very strong, 2,893,809.61 radioactivity measured in Gamma Ray American Petroleum Institute Units caused by its uranium concentration. Typically it can have arsenic, potassium, barium, iron, magnesium and calcium impurities. The crystal structure has a strong hydrogen bonding, where the water molecules are distributed in pairs, held together by two symmetrically related hydrogen bonds. It is further described to have formed from uranyl silicate layers having the uranophane sheet anion-topology. Water molecules are in the lead interlayer ions' coordination structure and reinforce it with hydrogen bonding between the uranyl silicate sheets. Its crystal structure is described as mechanically stable and very isotropic, which is unexpected as layered structures tend to be very anisotropic. Its large mechanical isotropy can be explained due to the strong dual hydrogen bonding between the uranyl silicate sheets. Because of the hydrogen bonding, the bonding strength along the direction perpendicular to sheets and that along the other directions are similar.

Formation

Carbonate and fluoride complexes play a major role in the formation of the mineral. Hydrothermal solutions reacting with uranium-bearing metamictized minerals form uranous fluoride complexes. These complexes are predominant at pH 4 and in reducing atmosphere. As the fluids pass through fractures, approaching the surface, the pH and the oxygen fugacity increases due to the loss of volatile components. Under these conditions, the uranous fluorides go through metamorphosis, becoming uranyl fluoride complexes. The fluorine ion activity decays due to the precipitation of fluorite and the dilution of the hydrothermal solutions, which both contribute to temperature decrease. When these conditions meet, uranyl-carbonate complexes are favored, which, when they combine with lead and silica, form kasolite. As lead is neither radiogenic nor a uranium substitute, the source of lead in the mineral structure might originate from galena's mineralization.

Occurrences and localities

Kasolite is an oxidation product of uraninite. This is the reason why it can usually be found in oxidized uranium deposits. In Kasolo, Congo, the mineral occurs in association with torbernite, curite, dewindtite and uraninite, and in Nabarlek, Australia, it can be found in association with rutherfordine, sklodowskite and curite as well. The top distributors of the mineral are Congo, Gabon, Germany, England, France, Australia, Canada, Mexico and the United States, however, there are numerous minor distributors as well.