Sodium heptasilicate; a high-pressure silicate with six-membered rings of tetrahedra interconnected by SiO 6 octahedra; (Na 8 Si[Si 6 O 18 ])
1998; Mineralogical Society of America; Volume: 83; Issue: 5-6 Linguagem: Inglês
10.2138/am-1998-5-621
ISSN1945-3027
Autores Tópico(s)Luminescence Properties of Advanced Materials
ResumoPretulite is a new Sc phosphate with zircon-type structure from the phyllite-micaschist hosted hydrothermal lazulite-quartz veins in the Lower Austroalpine Grobgneis complex, eastern Austria.The new species is the Sc-dominant analogue of xenotime-(Y) and occurs as an accessory mineral in all investigated lazulite specimens, forming anhedral to euhedral crystals up to 200 m long with the dominant form {211}.It is associated with lazulite, fluorapatite, chlorapatite, quartz, muscovite, clinochlore, paragonite, kyanite, pyrophyllite, augelite, wardite, hydroxylherderite, goyazite, florencite-(Ce), xenotime-(Y), bearthite, rutile, pyrite, corundum, and an AlO(OH)-phase.Pretulite is translucent to transparent with an adamantine luster, colorless to pale pink, uniaxial positive with ϭ 1.790 (5), ⑀ ϭ 1.86 (1) and shows a weak orange fluorescence at 254 nm as well as a bright blue cathodoluminescence.Mohs hardness is about 5. The empirical formula for pretulite (based on four O atoms) is (Sc 0.98 Y 0.02 ) 1.00 P 1.00 O 4.00 .It contains variable amounts of Y with Y/(Y ϩ Sc) ϭ 0.5 Ϫ 3.2 mol% and traces of Yb, Er, and Dy.The space group is I4 1 /amd with a ϭ 6.589 (1) A ˚, c ϭ 5.806 (1) A ˚, V ϭ 252.1 (1) A ˚3, d calc ϭ 3.71 g/cm 3 , Z ϭ 4. The four strongest lines in the X-ray powder pattern are d 200 ϭ 3.293 (100), d 112 ϭ 2.4636 (42), d 312 ϭ 1.6927 (45), d 332 ϭ 1.3697 (15) A ˚.The crystal structure was refined using 108 reflections to R(F o ) ϭ 0.018.Cell parameters and average M-O distances confirm a small amount of Y substituting Sc in the eightfold-coordinated M-position.The formation of pretulite is attributed to a moderate enrichment of Sc in lazulite-rich domains of the veins (about 180 ppm) and the inability of the accompanying minerals to incorporate larger quantities of Sc in their crystal structure.
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