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Piergorite-(Ce) is a new mineral found at Tre Croci, Vetralla, Italy with simplified formula Ca 8 Ce 2  (Al 0.5 Fe 3+   0.5 ) Σ1 (□,Li,Be) 2 Si 6 B 8 O 36 (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 α  = 1.717 (1), n β  = 1.728 (1), and n γ  = 1.735 (1), 2V meas  = 68(2)°, X = b, and Z ^ c = 7(1)°. Crystals show tabular habit and a very good {010} cleavage; twinning along the (301̄) plane produces “L” forms. The three strongest lines in the simulated powder diffraction pattern (d obs , I, hkl) are: 2.65 Å, 100.0, (213, 4̄13); 1.91 Å, 48.3, (223, 4̄23, 821); 2.90 Å, 44.9, (6̄03, 6̄12). 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) Å 3 , Z = 2] and was refined to a final R obs  = 0.059 for 6480 F o  with I o  &gt; 3σ(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.

american mineralogist

The crystal structure of piergorite-(Ce), Ca 8 Ce 2 (Al 0.5  Fe 3+   0.5 ) Σ1 (□,Li,Be) 2 Si 6 B 8 O 36 (OH,F) 2 : A new borosilicate from Vetralla, Italy, with a modified hellandite-type chain