BUSSYITE-(Ce), A NEW BERYLLIUM SILICATE MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC

Abstract

Bussyite-(Ce), ideally (Ce,REE,Ca) 3 (Na,H 2 O) 6 MnSi 9 Be 5 (O,OH) 30 (F,OH) 4 , a new mineral species, was found in the Mont Saint-Hilaire quarry, Quebec. The crystals are transparent to translucent, pale pinkish orange in color, with a white streak and vitreous luster. Bladed crystals are prismatic, having forms {111} and {101}; they are elongate on [101], up to 10 mm in length. Associated minerals include aegirine, albite, analcime, ancylite-(Ce), calcite, catapleiite, gonnardite, hydrotalcite, kupletskite, leucophanite, microcline, nenadkevichite, polylithionite, serandite and sphalerite. Bussyite-(Ce) is monoclinic, space group C 2/ c , with unit-cell parameters refined from powder-diffraction data: a 11.654(3), b 13.916(3), c 16.583(4) A, β 95.86(2)°, V 2675.4(8) A 3 and Z = 4. Electron-microprobe and secondary-ion mass spectrometric analyses give the average and ranges: Na 2 O 7.63 (9.40–6.62), K 2 O 0.05 (0.13–0.0), BeO 8.33 (SIMS), CaO 5.35 (5.95–4.17), MgO 0.03 (0.11–0.00), MnO 2.49 (3.00–1.71), Al 2 O 3 0.82 (1.54–0.39), Y 2 O 3 1.97 (2.68–1.51), La 2 O 3 2.65 (3.11–2.16), Ce 2 O 3 9.77 (11.22–8.15), Pr 2 O 3 1.23 (1.49–0.74), Nd 2 O 3 4.54 (5.13–3.91), Sm 2 O 3 0.99 (1.25–0.68), Eu 2 O 3 0.010 (0.25–0.0), Gd 2 O 3 1.03 (1.23–0.81), SiO 2 38.66 (39.94–37.66), ThO 2 3.31 (4.59–2.12), F 3.67 (6.39–2.54), S 0.03 (0.08–0), H 2 O 4.12 (determined from crystal-structure analysis), for a total of 95.21 wt.%. The empirical formula based on the crystal-structure analysis, ideally showing 34 anions, is: 4{(Ce 0.823 Nd 0.373 Y 0.242 Th 0.173 Pr 0.103 Sm 0.079 Gd 0.078 Eu 0.008 ) ∑1.879 (Ca 0.775 La 0.225 ) ∑1 [Na 3.000 (H 2 O) ∑2.500 Ca 0.544 K 0.015 ] ∑6.055 (Mn 0.485 Na 0.402 Mg 0.012 ) ∑0.899 (Si 8.897 Be 4.605 Al 0.222 ) ∑13.724 O 30 [F 2.67 (OH) 1.33 ] ∑4 }. The structure has been refined to an R index of 4.0% for 1134 unique, observed reflections. The structure has two chemically distinct layers parallel to (101): (1) a layer of [(Si,Be)O 4 ] tetrahedra, and (2) a layer of Ce–, Ca–, Mn–, Na–(O,F) polyhedra. Layers are cross-linked through shared O and F atoms. Significant amounts of OH and H 2 O are present, as indicated in the IR spectrum and crystal-structure analysis for the Na-poor hydrated phase. One instance of a rarer Na-rich phase is observed. Based on the electron-microprobe data, the formula for the rarer Na-rich anhydrous phase is 4{(Ce 1.047 Nd 0.466 La 0.323 Y 0.306 Pr 0.131 Sm 0.102 Gd 0.084 Eu 0.009 Th 0.042 ) ∑2.510 (Na 6.645 Ca 0.701 ) ∑7.346 (Si 9.521 Be 4.772 ) ∑14.293 O 29.251 F 4.749 }. Comparing the structure of bussyite-(Ce) to that of other beryllium silicates, the topology of the layer of tetrahedra most strongly resembles that of semenovite-(Ce) and the closely related mineral harstigite.