The crystal structure of piergorite-(Ce), Ca8Ce2(Al0.5 Fe3+0.5) Σ 1(□,Li,Be)2Si6B8O36(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+)∑1(□,Li,Be)2Si6B8O36(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, nα = 1.717 (1), nβ = 1.728 (1), and nγ = 1.735 (1), 2Vmeas = 68(2)°, X = b, and Z ^ c = 7(1)°. Crystals show tabular habit and a very good {010} cleavage; twinning along the (30-1) plane produces “L” forms. The three strongest lines in the simulated powder diffraction pattern (dobs, I, hkl) are: 2.65 Å, 100.0, (213, -413); 1.91 Å, 48.3, (223, -423, 821); 2.90 Å, 44.9, (-603, -612). The structure was solved by Patterson synthesis from X-ray diffraction data [monoclinic, space group P2/a, a = 28.097(3) Å, b = 4.777(1) Å, c = 10.236(2) Å, β = 96.81(1)°, V = 1364.2(7) Å3, Z = 2] and was refined to a final Robs = 0.059 for 6480 Fo with Io > 3σ (Io). 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.