The crystal structure of piergorite-(Ce), Ca8Ce2(A10.5Fe3+0.5)Sigma1( , Li, Be)2Si6B8036(OH,F)2:A new borosilicate from Vetralla, Italy, with a modified hellandite type chain.

Piergorite-(Ce) is a new mineral found at Tre Croci, Vetralla, Italy with simplified formula Ca8Ce2 (Al0.5Fe0.53+)(Sigma 1)(square,Li,Be)(2)Si6B8O36(OH,F)(2). It occurs as strong intergrowths of small crystals, colorless to pale yellow, associated with sanidine, mica, magnetite, rutile, titanite, and other Th-U-REE bearing minerals, in miarolitic cavities of a syenitic ejectum. Piergorite-(Ce) is biaxial negative, n(alpha) = 1.717 (1), n(beta) = 1.728 (1), and n(gamma) = 1.735 (1), 2V(meas) = 68(2)degrees, X = b, and Z (boolean AND) c = 7(1)degrees. Crystals show tabular habit and a very good {010} cleavage; twinning along the (30 (1) over bar) plane produces "L" forms. The three strongest lines in the simulated powder diffraction pattern (d(obs), I, hkl) are: 2.65 angstrom, 100.0, (213, (4) over bar 13); 1.91 angstrom, 48.3, (223, (4) over bar 23, 821); 2.90 angstrom, 44.9, ((6) over bar 03, (6) over bar 12). The structure was solved by Patterson synthesis from X-ray diffraction data [monoclinic, space group P2/a, a = 28.097(3) angstrom, b = 4.777(1) angstrom, c = 10.236(2) angstrom, beta = 96.81 (1)degrees, V = 1364.2(7) angstrom(3), Z = 2] and was refined to a final R-obs = 0.059 for 6480 Fo with I-o > 3 sigma(I-o). The structure shows similarities with the hellandite group because Si and B tetrahedra form chains along c. Hellandite structure is characterized by a single chain of five-membered rings, whereas piergorite-(Ce) shows a double chain of five-membered rings interconnected by a single octahedral site to form a three-dimensional framework containing five independent eightfold-coordinated M sites and a partly occupied T-cavity.