Canadian Rockhound: Horváthite-(Y)

Horváthite-(Y)
Abstracted by Robert Gault

Horváthite-(Y), ideally NaY(CO₃)F₂, is a newly described mineral species from Mont Saint-Hilaire, Quebec. It was formerly designated as UK 104. The cotype specimens were collected in 1994 by Stephen and Janet Cares from a heavily altered section of a large pegmatite dike in the southeastern corner of the Poudrette quarry. While only a few milligrams of material were collected during the initial find, a considerable number of specimens have been collected in recent weeks. Associated species are microcline, aegirine, dawsonite, rhodochrosite, sodalite, natrolite, albite, serandite, siderite, taeniolite, catapleiite, astrophyllite, genthelvite, a franconite-group mineral, sphalerite and UK 96.

Horváthite-(Y) has two distinct habits: it occurs as (1) stacks of (100) plates, approximately 1 x 1 x 0.25 mm and as (2) blocky aggregates of radiating crystals 1-5 mm across with individual, striated, prismatic crystals up to 1 mm long on [010]. The aggregates are commonly 'wagon-wheel' or cylindrical in habit. Horváthite-(Y) is colourless to pale beige, translucent to transparent with a vitreous lustre. It is brittle, with very good {100} and good {010} cleavages. The mineral is relatively soft (Mohs hardness 4) and does not fluoresce in ultraviolet light. D_calc.3.613 g/cm³. It is biaxial negative, with indices of refraction a 1.457(1) b 1.543(1) and g 1.622(1); 2V_meas. = 82(1)º (for l = 590 nm). The mineral is weakly pleochroic with Y » Z pale brown, and X colourless. Horváthite-(Y) is orthorhombic, space group Pmcn, a 6.959(2) b 9.170(2) c 6.301(1) Å, V 402.1(2) Å³ and Z = 4.

Source:

Grice, J.D. & Chao, G.Y. 1997. Horváthite-(Y), rare-earth fluorocarbonate, a new mineral species from Mont Saint-Hilaire, Québec. Can. Min. 35:743-749.

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