Florencite-(La)

florencite-(La)

lanthanum

uraninite

coffinite

Images

Formula: LaAl3(PO4)2(OH)6
Anhydrous phosphate, plumbogummite group, alunite supergroup, lanthanum-bearing mineral
Crystal System: Trigonal
Specific gravity: 3.52 measured, 3.71 calculated
Hardness: 5 to 6
Colour: Colourless to pale yellow, white in aggregates
Weakly RADIOACTIVE
Environments

Sedimentary environments

Localities

At the type locality, the Shituru Mine, Likasi, Haut-Katanga, DR Congo, florencite-(La) occurs as euhedral, colourless to pale yellow crystals normally not exceeding 30 µm across in siltstones belonging to an intertidal facies and overlain by lagoonal dolostone. Associated minerals include apatite, clay minerals, dolomite, hematite, quartz, rutile and zircon (Mindat).

At Bangombé, Léboumbi-Leyou Department, Haut-Ogooué Province, Gabon, spontaneous fission reactions occurred in uranium deposits approximately 2 billion years ago and lasted 105 to 106 years, producing a substantial amount of fission products. The Bangombe fission reactor, discovered in 1985, is in sandstone at a depth of 12 m and consists of a 5 cm thick body of a massive uranium ore (the reactor's "core"), which is overlain by a 30 cm thick clay mantle.
Florencite-(La) and florencite-(Ce) have been identified in illite from the clay mantle and in sandstone beneath the reactor zone, respectively. Florencite-(Ce) is apparently unrelated to nuclear processes and occurs with monazite-(Ce), apatite, Ti02 (probably anatase), zircon and illite. Grains of florencite-(Ce) contain inclusions of thorite, chalcopyrite, and galena.
Florencite-(La) was found 5 cm from the "core" of the reactor and contains inclusions of galena and uranium-titanium -bearing phases. Secondary uraninite and coffinite have precipitated on some of the florencite grains.
Grains of florencite contain numerous inclusions of thorite, chalcopyrite and galena. Though monazite grains are heavily corroded and altered, they are not replaced by florencite. In areas where monazite and florencite are in physical contact, the latter is an overgrowth on monazite. Apatite grains are corroded; they have not been seen in physical contact with florencite.
The florencite-(La)-bearing sample from the reactor zone consists of illite with veinlets and aggregates of authigenic (formed in place) ferromagnesian chlorite. One of the numerous fractures divides this sample into two portions: one contains florencite, galena, nodules of solid bitumens associated with titanium-, zirconium- and vanadium- rich phases, and uranium minerals coffinite and uraninite. The other portion contains abundant pyrite and detrital zircon dispersed within illite. Florencite is embedded in the brownish illite and occurs as a train of elongated overgrowths of colourless to yellowish rhombohedral crystals up to 110 microns long and 50 microns wide. Some florencite grains contain inclusions of galena and vanadium- and titanium-bearing phases. Uraninite and coffinite precipitated on florencite and replaced it along fractures. Precipitation of uraninite on florencite and adjacent illite a few centimetres outside the reactor core suggests that this is a secondary uraninite (AM 81.1263-1269).

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