Priscillagrewite-(Y), (Ca₂Y)Zr₂Al₃O₁₂: A new garnet of the bitikleite group from the Daba-Siwaqa area, the Hatrurim Complex, Jordan

Priscillagrewite-(Y), ideally (Ca2Y)Zr2Al3O12 (Ia3¯ ¯ d, a = 12.50 Å, V = 1953.13 Å3, Z = 8), a new member of the garnet supergroup and bitikleite group, was discovered in a fluorapatite layer (meta-phosphorite) hosted by varicolored spurrite marble in the Daba-Siwaqa area of the Transjordan plateau south of Amman, central Jordan. The Daba-Siwaqa area is the largest field of the Hatrurim Complex pyrometamorphic rocks distributed along the rift of the Dead Sea. Priscillagrewite-(Y) and other accessory minerals (such as members of the brownmillerite-srebrodolskite series, fluormayenite, lakargiite, baghdadite, hematite, sphalerite, zincite, garnet of the andradite-grossular series, tululite, vapnikite, minerals of the lime-monteponite series and members of the magnesiochromite-zincochromite series, cuprite, and Y-bearing and Y-free perovskite) are distributed irregularly in varicolored spurrite marble. The empirical formula of priscillagrewite-(Y), based on 12 O atoms, is Ca2.19Y0.65Ce0.033+Nd0.033+Gdd0.023+Dy0.023+$\left(\mathrm{Ca}_{2.19} \mathrm{Y}_{0.65} \mathrm{Ce}_{0.03}^{3+} \mathrm{Nd}_{0.03}^{3+} \mathrm{Gdd}_{0.02}^{3+} \mathrm{Dy}_{0.02}^{3+}\right.$Er0.023+Yb0.023+La0.013+Sm0.013++ς3.00Zr1.79Ti0.134+Sb0.075+U0.016+ς2.00Al1.70Fe1.213+Si0.04P0.04S+ς2.99O12.$\left.\mathrm{Er}_{0.02}^{3+} \mathrm{Y} \mathrm{b}_{0.02}^{3+} \mathrm{L} \mathrm{a}_{0.01}^{3+} \mathrm{Sm}_{0.01}^{3++}\right)_{\Sigma 3.00}\left(\mathrm{Zr}_{1.79} \mathrm{Ti}_{0.13}^{4+} \mathrm{Sb}_{0.07}^{5+} \mathrm{U}_{0.01}^{6+}\right)_{\Sigma 2.00}\left(\mathrm{Al}_{1.70} \mathrm{Fe}_{1.21}^{3+} \mathrm{S} \mathrm{i}_{0.04} \mathrm{P}_{0.04}^{\mathrm{S}+}\right)_{\Sigma 2.99} \mathrm{O}_{12}.$ A good match was obtained for electron backscatter diffraction (EBSD) patterns with a garnet model having a = 12.50 Å. The new garnet forms idiomorphic, isometric crystals up to 15 μm in size. It is transparent and has pale yellowish tinge, and its luster is vitreous. Priscillagrewite-(Y) is isotropic: N = 1.96 based on the Gladstone-Dale calculation using a = 12.50 Å and the empirical formula. The Mohs hardness is about 7-7.5. Density calculated from the empirical formula is 4.48 g/cm3. Raman spectrum of priscillagrewite-(Y) is similar to those of other minerals of the bitikleite group and contains the following bands (cm-1): 150, 163, 240, 269, 289, 328, 496, 508, 726, and 785. The strongest lines of the calculated powder diffraction data are as follows [(hkl) dhkl (I)]: (422) 2.552 (100), (642) 1.670 (96), (420) 2.795 (84), (400) 3.125 (72), (200) 4.419 (35), (640) 1.733 (32), and (1042) 1.141 (25). Priscillagrewite-(Y) is interpreted to be a relic of the high-temperature association formed in the progressive stage at the peak pyrometamorphism conditions when temperature could have reached close to 1000 °C.