Oxides and hydroxides: Fergusonite

Diagnostic card.

Y Nb O ₄
Tsingonia tetragonal
Hardness 5,5-6,5
Specific weight 5,4
Cracked shell
Color from brown to black
Color in powder light brown
Glitter glass

This mineral has two clearly different modifications - tetragonal and monoclinic. Crystals are rare. The form is usually shortened prisms and bipyramids, revealing the symmetry of the bipyramidally-tetragonal class. Color - from brown and dark gray to yellow; Shine - fat-resin. The fine grain edges do not melt or melt. After calcination it becomes lighter, becomes opaque.

Theor. Composition for YNbO4: Y2O3 - 45.95; Nb2O5, 54.05. Niobium is replaced by Ta, Ti. The Y content usually ranges from 63-75% of the total Y + TR; Y is replaced by rare-earth elements, mainly Yb, to a lesser degree Dy, Gd, Er, also Nd, Ce, Sm]. Ca, Th and U substitute Y to an insignificant degree. The content of CaO is 1-3%, UO2 1-4% (sometimes up to 8%); ThO2 0.5-5%); The content of Ta2O5 is insignificant in natural fergusonites, but sometimes reaches 4-9%. Harakt. Allotment. Solid precipitates, granular aggregates, individual grains and crystals.

For the most part, metamict X-ray amorphous fergusonite is observed in nature; Rarely completely or partially non-methactical. Dimorphen: a-fergusonite-tetrag. from. C62h, -I4ha; Z = 4. b-fergusonite monocle. from. (Pseudotetragonal). The composition and properties are indistinguishable. Tetrag-dipyramid. Cl. C4h - 4 / m (L4PC); A: c-1: 1,464. The crystals are barrel-shaped, lenticular, spindle-shaped, less often prismatic or tabular (001), usually imperfect; The faces are often deformed, corroded, matte, the edges are rounded, the crystals are covered with crusts of undetermined change products.

Cleavage according to (001) is quite distinct, quite imperfect in (111). Microhardness 683-897 kg / mm2 at a load of 100-200 g, 689-757 at 100 g. at. 5.6-6.0 (calculated for YNbO4 - 5.58), increases with increasing tantalum content; With a change in the mineral ud. Weight is significantly reduced. Bl. Resin, matte, in the fracture glass, semimetallic to diamond-like. In thin fragments shines brown or dark brown color. The infrared spectrum is characterized by an absorption band in the region of 833-714 cm-1 and a transmission maximum of about 950 cm-1.

In sl. In the proh. Light brown, orange-yellowish or reddish-brown, light yellowish, often heterogeneously colored. Usually isotropic (the result of metamicture). Sometimes anisotropic areas can be distinguished in the mass of an isotropic mineral. Anisotropic nuclei and isotropic outer zones were observed. Uniaxial (-). Optical properties of anisotropic nonmetametric tetragonal fergusonite from the Urals: ng> nm = 2.28; Np = 2.18 (Na); Ng - pr about 0,08 and Yakutia: n0 = 2,28; Ne = 2Д8; N0 - ne -. 0.08; The Ural biaxial with 2V = 34 ^o . The refractive indices are 2.00 - 2.19. In the polisher, sl. In reflection. Light gray, creamy-gray, sometimes heterogeneous in color. The Ural nonmetamict fergusonite is distinctly anisotropic, characterized by strong light-brownish internal reflexes.

Insoluble in HCl and HNO3, decomposes by boiling at the end. H2SO4 and HNO3; In the cold - in HF with the isolation of hard soluble yttrium fluoride; Completely decomposes upon fusion with KHSO4. In polish, according to Barsanov, it is very difficult to etch HF; HF acts quickly, thus forming a milky white film; From the boiling H2SO4 a weak brown spot appears; Sometimes etched with a mixture of NH4F + HC1, a brown or iridescent spot is formed.

Place of Birth.
It occurs mainly in granite pegmatites (Iterby in Sweden, Amelia-Korthouse in Virginia, Aduy in the central Urals, Madavasca in Ontario, etc.). There are also several alluvial deposits (Rakwana in Sri Lanka). A rare species containing titanium (rizorite), is found near Rizor in Norway. Monoclinic species found in one locality in Central Asia.

Fergusonite- (Y). Niriella, Ratnapura (rn), Sri Lanka. Photo: © А.А. Evseev.

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