By John Betterton

Thorium, Th, is a relatively rare metal in the Earth’s crust with an average abundance of roughly 5.6 ppm. World production currently stands at around 5000 tonnes per year with Brazil, India, Turkey, Australia and the USA the major produces. The monazite group of phosphate minerals are the current source of the world’s supply of this element. Most of the older uses for this element have been discontinued due to its radioactivity and is still used gas tungsten arc welding.

rock n gem magazine issue 68
rock n gem magazine issue 68

Other uses are in heat-resistant ceramics, quality lenses, catalysts, themionic emitting devices, scientific equipment and as a possible source of nuclear fuel. Most of its minerals can be found among the oxides and silicate groups.
Thorianite, ThO2, is a common accessory mineral found in granitic pegmatites, carbonatites and serpentinites and in various detritial heavy mineral concentrates. It is associated with thorite, zircon, ilmenite, cassiterite, allanite group minerals, diopside, spinel and phlogopite. The mineral occurs as rough cubes and as rounded grains. Twinned crystals are frequent. It has a poor cleavage and an uneven to subconchoidal fracture. The mineral is brittle with a hardness of 6.5 to 7 when unaltered. Specific gravity is high at 9.9 and is radioactive in nature. It is black, brownish black, dark grey, to reddish brown in colour and may have a bronzy tarnish in some examples. The lustre is variable from submetallic when fresh, altering to resinous or horn-like with a grey to greenish grey streak. Thorianite is opaque and transparent only in thin fragments. Dark colourations, high specific gravity, radioactivity and mineralogical environment contribute to its identify. This species is widespread with numerous known locations in Madagascar, South Africa, Russia, USA, Canada, Brazil, China and in many other countries.

Thorutite, (Th,U,Ca)Ti2((O,OH)6, is a monoclinic species that is always found in the metamict state and possesses a conchoidal fracture with a hardness of 5 to 6. The specific gravity is around 5.6. It occurs as short prismatic crystals up to about 2 cm in size. This mineral is translucent with a black colour that is brown on thin edges. The streak is pale brown with a resinous lustre. Thorutite occurs in veins syenite with microcline, thorite, zircon, calcite, barite and galena. The above cited physical and chemical properties together with x-rays aid in identification. About eight separate locations are know for this uncommon thorium-bearing species and include those in Australia, Brazil, India, Kyrgyzstan, Namibia, Russia, Sri Lanka and the USA.

Tuliokite, Na6BaTh(CO3)6.6H2O, is an unusual mineral compositionally and occurs in hydrothermal veins in syenite pegmatites associated with nepheline, sidorenkite, vinogradovite, villiaumite, microcline, pirssonite, shortite, trona, thermonatrite, natron, natrolite and aegirine. Tuliokite crystallises in the hexagonal system and forms prismatic and rhombohedral crystals up to about 4 mm in size. It is pale to dark grey in colour with a white streak and vitreous lustre.

This mineral is radioactive, brittle and has a hardness of 3 to 4. Specific gravity is 3.1. The bizarre chemistry, radioactivity and restricted number of locations known are enough for its recognition. The single site for this species is the Kirovskii apatite mine, Kukisvumchorr Mt, Khibiny Massif, Russia.

Cheralite, (Ca,Ce)(Th,Ce)(PO4)2, is to be found in carbonatites and disseminated kaolinized pegmatites and is accompanied by tourmaline supergroup members, chrysoberyl, zircon, quartz and huttonite. The mineral is monoclinic, frequently metamict in character and is usually found as subhedral to anhedral massive aggregates. It is translucent and pale to dark green, honey-brown to yellow-brown in colour with a white streak. The lustre is variable from resinous, greasy to vitreous. It possesses several distinct to difficult cleavages, and a poor parting. The fracture is uneven and is brittle. Its hardness is 5 and the specific gravity is 5.2. This species is radioactive. The above cited properties and chemistry help towards characterisation.Specimens can be found in several countries including the USA, Germany, Italy, Finland, the Czech Republic, Austria, Australia and Argentina. Other lesser sites are known for this species.
Thorium molybdates were unknown to exist until 2014, and now we have two species described below from the same locality. X-rays and chemical analysis are best for identification for these two minerals.

Ichusaite, Th(MoO4)2.3H2O, is the first thorium molybdate mineral species to be discovered and crystallises in the monoclinic system. Its habit is as aggregates of thin tabular crystals. The mineral is colourless with a pearly to adamantine lustre. The streak is white and transparent.

A perfect cleavage is present. The calculated specific gravity is 4.2. Ichusaite is found intimately with nuragheite within vugs of quartz veins associated with muscovite and xenotime-(Y).
It has been found as a probable alteration product of molybdenite ore under basic conditions at the Mo-Bi deposit at Su Seinargiu, Cagliari, Sardinia, Italy.

Nuragheite, Th(MoO4)2.H2O, is the second thorium molybdate mineral to be reported in nature. It is orthorhombic and forms minute thin tabular crystals.
This mineral is colourless with a pearly to adamantine lustre. It is brittle with a perfect cleavage. Other data is not known. Nuragheite occurs with muscovite, xenotime-(Y) and ichusaite.
This species occurs as a probable alteration product of molybdenite ore under basic conditions in the Mo-Bi deposit at Su Seinargiu, Cagliari, Sardinia, Italy.

Ekanite, Ca2ThSi8O20, crystallises in the tetragonal system and forms poorly developed pyramidal crystals to 1 cm in size and are usually striated. Several different crystal habits are known along with massive and clustered aggregates.
Distinct to indistinct cleavages are present and the fracture is irregular in form.

This mineral is brittle and radioactive. It is
transparent to translucent, and is colourless, light to dark green, yellow-green, yellowish-brown, straw yellow or dark red in colour.
The streak is white and the lustre is vitreous. Its hardness is 4.5 to 5 and the specific gravity is 2.9. Chemistry, x-rays and physical properties are used for characterisation. Ekanite occurs in various geological environments including syenitic rocks, volcanic ejecta and as a detrital species. Fluorite, garnet supergroup members, quartz, microcline, clinopyoxenes, apatite supergroup members, plagioclase, hematite, zircon and titanite etc commonly accompany it. Specimens of this gorgeous mineral have been found in Sir Lanka, Burma, Canada, Italy, Germany, Portugal and USA.

Huttonite, ThSiO4, can be found in beach sands with scheelite, cassiterite, uranothorite, ilmenite, gold, cheralite, augite, and ningyoite. It occurs as flattened anhedral grains that are colourless, cream to pale yellow green. It is transparent to translucent and fluoresces dull white with a pink tinge under SW UV light. Distinct to poor cleavages are present with a conchoidal fracture.

The hardness is around 4.5. and the specific gravity is high at 7.1. It is radioactive in nature. The above physical properties together with x-rays are helpful to distinguish from the dimorph thorite.
A wide variety of locations are known for huttonite with New Zealand, Australia, Germany, Finland, Poland, Italy, Czech Republic, Sweden, USA, Russia among the best recognized.

Thorite, ThSiO4, is the dimorph of huttonite and crystallises in the tetragonal system but is commonly metamict. It generally is found as square
prisms or as pseudo-octahedral crystals to 8 cm in size. Massive and compact aggregates also exist. The mineral is nearly opaque and is yellow-orange to brownish yellow in colour with a light orange to dark brown streak. Its lustre is vitreous, resinous to greasy and is transparent in thin splinters. A distinct cleavage is present and the fracture is conchoidal in form. It is a brittle mineral with a hardness of 4.5 to 6.
The specific gravity is high between 6.6 and 7.3.
Thorite is paramagnetic and radioactive in nature.
The above physical properties and x-ray analysis are used to identify it from huttonite.
It occurs in as an accessory speciespecies in fesic rocks and associated pegmatites with zircon, monazite, gadolinite, fergusonite group minerals and uraninite etc. Good samples have been obtained from the following nations: Norway, Finland, Germany, Sweden, Austria, Czech Republic, France, Italy, Madagascar, Argentina, USA, Canada, Australia, Brazil, China, Japan, Russia, Kazakhstan, Namibia and South Africa.
The element silicon and its numerous and abundant minerals will be described in the next series of future articles starting with the silica minerals.


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