Huttonite
Unit cell of huttonite
General
Category	Silicate mineral
Formula (repeating unit)	ThSiO4
IMA symbol	Ht[1]
Strunz classification	9.AD.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 6.77 Å, b = 6.96 Å c = 6.49 Å; β = 104.99°; Z = 4
Identification
Formula mass	324.12 g/mol
Color	Colorless, cream, pale yellow
Crystal habit	Prismatic, flattened; typically as anhedral grains
Cleavage	Distinct along [001], indistinct along [100]
Fracture	Conchoidal
Mohs scale hardness	4.5
Luster	Adamantine
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	7.1
Optical properties	Biaxial (+)
Refractive index	nα = 1.898, nβ = 1.900, nγ = 1.922
Birefringence	δ = 0.0240
2V angle	25°
Dispersion	r < v (moderate)
Ultraviolet fluorescence	Dull white (under shortwave)
Other characteristics	Radioactive
References	[2][3][4]

Huttonite is a thorium nesosilicate mineral with the chemical formula ThSiO₄ and which crystallizes in the monoclinic system. It is dimorphous with tetragonal thorite, and isostructual with monazite. An uncommon mineral, huttonite forms transparent or translucent cream–colored crystals. It was first identified in samples of beach sands from the West Coast region of New Zealand by the mineralogist Colin Osborne Hutton (1910–1971).^[5] Owing to its rarity, huttonite is not an industrially useful mineral.

Occurrence

Huttonite was first described in 1950 from beach sand and fluvio-glacial deposits in South Westland, New Zealand, where it was found as anhedral grains of no more than 0.2 mm maximum dimension. It is most prevalent in the sand at Gillespies Beach, near Fox Glacier,^[5][6] which is the type location, where it is accompanied by scheelite, cassiterite, zircon, uranothorite, ilmenite and gold. It was found at a further six nearby locations in less plentiful amounts.^[7] Huttonite was extracted from the sands by first fractionating in iodomethane and then electromagnetically. Pure samples were subsequently obtained by handpicking huttonite grains under a microscope. This was accomplished either in the presence of short wave (2540 Å) fluorescent light, where the dull white fluorescence distinguishes it from scheelite (fluoresces blue) and zircon (fluoresces yellow), or by first boiling the impure sample in hydrochloric acid to induce an oxide surface on scheelite and permitting handpicking under visible light.^[7]

Hutton suggested the huttonite contained in the beach sand and fluvio-glacial deposits originated from Otago schists or pegmatitic veins in the Southern Alps.^[7]

In addition to New Zealand, huttonite has been found in granitic pegmatites of Bogatynia, Poland,^[8] where it associated with cheralite, thorogummite, and ningyoite; and in nepheline syenites of Brevik, Norway.^[9]

Physical properties

Huttonite typically occurs as anhedral grains with no external crystal faces. It is usually colorless but also appears in colors; such as cream and pale yellow. It has a white streak. It has a hardness of 4.5 and exhibits distinct cleavage parallel to the c-axis [001] and an indistinct cleavage along the a-axis [100].

Structure

Huttonite is a thorium nesosilicate with the chemical formula ThSiO₄. It is composed (by weight) of 71.59% thorium, 19.74% oxygen, and 8.67% silicon. Huttonite is found very close to its ideal stoichiometric composition, with impurities contributing less than 7% mole fraction. The most significant impurities to be observed are UO₂ and P₂O₅.^[10]

Atomic environment along a SiO₄–ThO₅ chain (parallel to the c axis)

Huttonite crystallizes in the monoclinic system with space group P2₁/n. The unit cell contains four ThSiO₄ units, and has dimensions a = 6.784 ± 0.002Å, b = 6.974 ± 0.003Å, c = 6.500 ± 0.003Å, and inter–axis angle β = 104.92 ± 0.03^o. The structure is that of a nesosilicate — discrete SiO₄²⁻ tetrahedra coordinating thorium ions. Each thorium has coordination number nine. Axially, four oxygen atoms, representing the edges of two SiO₄ monomers on opposite sides of the thorium atom, form a (–SiO₄–Th–) chain parallel to the c axis. Equatorially, five nearly planar oxygen atoms representing vertices of distinct silicate tetrahedra coordinate each thorium. The lengths of the axial Th–O bonds are 2.43 Å, 2.51 Å, 2.52 Å, 2.81 Å, and of the equatorial bonds, 2.40 Å, 2.41 Å, 2.41 Å, 2.50 Å, and 2.58 Å. The Si–O bonds are nearly equal, with lengths 1.58 Å, 1.62 Å, 1.63 Å, and 1.64 Å.^[11]

Huttonite is isostructural with monazite. Substitution of the rare-earth elements and phosphorus of monazite with thorium and silicon of huttonite can occur to generates a solid solution. At the huttonite end-member, continuous rare-earth substitution of thorium of up to 20% by weight has been observed. Thorium substitution in monazite has been observed up to 27% by weight. Substitution of PO₄ for SiO₄ also occurs associated with the introduction of fluoride, hydroxide, and metal ions.^[12]

Huttonite is dimorphic with thorite. Thorite crystallizes in a higher symmetry and lower density tetragonal form in which the thorium atoms coordinate to one less oxygen atom in an octahedral arrangement. Thorite is stable at lower temperatures than huttonite; at 1 atmosphere, the thorite–huttonite phase transition occurs between 1210 and 1225 °C. With increasing pressure the transition temperature increases. This relatively high transition temperature is thought to explain the relative rarity of huttonite on the Earth's crust.^[13] Unlike thorite, huttonite is not affected by metamictization.

References

External links

• American Mineralogist Crystal Structure Database entry