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Thucholite is a pyrobitumen mineraloid notable for incorporating rare elements, including thorium and uranium into its amorphous structure. Like other types of pyrobitumen, thucholite is an infusible and insoluble naturally occurring hydrocarbon. The uniqueness of thucholite stems from the presence of thorium and uranium, and the mineral is often found radioactive due to the isotopes of thorium and uranium that are present. The origin of thucholite in the Earth is disputed, with some researchers claiming that thucholite is formed by the polymerization of methane migrating through uranium-containing rock, while others argue that it is formed, like petroleum, from plant matter, and later contaminated with heavy elements leached from surrounding uranium-containing rock.

Study
Thucholite is a rare mineral and, to this day, is little studied. The first detailed description of the mineral was made in 1928 by H.V. Ellsworth of the Canadian Geological Survey, who analyzed the volatile gas composition of a sample of the mineral unearthed at the Besner Mine in the Parry Sound District of Ontario and coined the name thucholite as an abbreviation of the names of the elements found in it: Th, U, C and H. An increasing number of studies focused on or mentioning the mineral were published in the mid-twentieth century, as the United States Geological Survey and the United States Atomic Energy Commission funded research into radioactive ores. Further study by geologists revealed the composition of the ash, which is left behind after burning.

Composition
The composition of thucholite varies widely, due in part to the mineral’s loose definition. It has been quantified as approximately 50% carbon, 25% volatile gases, including water vapor, carbon dioxide, and methane, and 25% ash composed of various rare earth oxides, especially yttrium oxide, uranium oxide, and thorium oxide. Indeed, researchers often cite the presence of uranium oxide (UO2) and thorium oxide (ThO2) within a hydrocarbon, regardless of the hydrocarbon’s source, as the defining feature of thucholite, even though the concentrations of the two oxides, and their ratio to each other, may vary from specimen to specimen. (U3O8) has been known to substitute for (UO2) in uraninite and therefore in thucholite samples that receive their uranium content from said uraninite. Indeed, depending on the form of the uranium in a thucholite sample's surrounding rock, the uranium in the sample may also be in the form of coffinite, various hexavalent uranium minerals, or organo-uranium complexes.

Properties
Thucholite, as a form of pyrobitumen, has the properties of other hydrocarbons. It is jet-black and opaque, even when cut into the thinnest of layers. As a mineraloid, it is amorphous and has a conchoidal fracture. Thucholite is rarely found in large pieces, but rather in small nodules, 0.1 to 1.6 cm in diameter and roughly round in shape, embedded in other minerals. What makes the mineral special, of course, is its tendency to be radioactive due the (ThO2) and (UO2) present. 232Th, the only isotope of thorium present in significant quantity on Earth, and 235U, the rarer but nonetheless extant isotope of uranium, are the source of this radioactivity.

Geological Setting
The first detailed description of thucholite was made by H.V. Ellsworth at the Besner Mine in Ontario, Canada, where it was found in pegmatite dikes in 1921. Thucholite was found in close association with uraninite, calciosamarskite, and allanite and, together with other minerals, were embedded in feldspar, mica and quartz in the form of irregularly rounded nodules. The proximity of thucholite in situ to uraninite led geochemists to conclude that surrounding uraninite is the source of the uranium in thucholite. Ellsworth had suggested that thucholite may, in fact, actively substitute for uraninite in uranium ores.

According to a USGS study in 1963, uranium-bearing hydrocarbons can be found throughout the contiguous United States and in Saskatchewan, Canada, the Isle of Man, Great Britain, Australia, and Russia. However, according to the report, thucholite is rarely found in deposits that can be classified as veins, and none of the existing veins are important sources of uranium ore.