Pretulite 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 mm 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
lustre, colorless to pale pink, uniaxial positive with w
= 1.790 (5), e = 1.86 (1) and shows a weak orange
fluorescence at 254 nm as well as a bright blue cathodoluminescence. Mohs
hardness is ca. 5. The empirical formula for pretulite (based on four oxygens)
is (Sc0.98Y0.02)1.00P1.00O4.00. It contains variable amounts of Y with
Y/(Y+Sc) = 0.5-3.2 mol% and traces of Yb, Er and Dy. The space group is
I41/amd with a = 6.589 (1) Å, c = 5.806 (1) Å, V = 252.1 (1)
Å3, dcalc = 3.71 g/cm3, Z = 4. The strongest lines in the X-ray powder
pattern are d200 = 3.293(100), d112 = 2.4636(42), d202 = 2.1777(14), d301
= 2.0546(14), d312 = 1.6927(45), d400 = 1.6470(14), d332 = 1.3697(15) Å.
The crystal structure of pretulite was refined using 108 reflections to
R(Fo) = 0.018. Cell parameters and average M-O distances confirm a small
amount of Y substituting Sc in the eight-coordinated M-position. The formation
of pretulite is interpreted to be due to a moderate enrichment of Sc in
lazulite-rich domains of the veins (ca. 180 ppm) and the inability of the
accompanying minerals to incorporate larger quantities of Sc in their crystal
structure. The name is after the mountain Pretul, Fischbacher Alpen, Styria,
SEM-photograph of a distorted, euhedral pretulite crystal
with the dominant form (211), small (100) and rounded (111).
This crystal was used for structure refinement and is mounted
with epoxy. Sample HK1A (Höllkogel).
BSE-image of a sectioned pretulite grain, showing
chemical inhomogenity. Brighter areas are enriched
in Y, white areas are fluorapatite.
Sample HK1A (Höllkogel).