Istituto di Geoscienze e Georisorse     
Boiocchi M., Callegari A., Ottolini L. The crystal structure of piergorite-(Ce), Ca8Ce2 Al0.5Fe3+0.5)ε1( ,Li,Be)2Si6B8036(OH,F)2: A new borosilicate from Vetralla, Italy, with a modified hellandite type chain. In: American Mineralogist, vol. 91 (7) pp. 1170 - 1177. Mineralogical Society of America, 2006.
Piergorite-(Ce) is a new mineral found at Tre Croci, Vetralla, Italy with simplified formula Ca8Ce2 (Al0.5Fe3+0.5)Σ1(■,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 (301-) plane produces "L" forms. The three strongest lines in the simulated powder diffraction pattern (dobs, I, hkl) are: 2.65 , 100.0, (213, 4-13); 1.91 , 48.3, (223, 4-23, 821); 2.90 , 44.9, (6-03, 6-12). The structure was solved by Patterson synthesis from X-ray diffraction data [monoclinic, space group P2/a, a = 28.097(3) , b = 4.777(1) , c = 10.236(2) , β = 96.81(1), V = 1364.2(7) 3, Z = 2] and was re ned to a nal 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 ve-membered rings, whereas piergorite-(Ce) shows a double chain of ve-membered rings interconnected by a single octahedral site to form a three-dimensional framework containing ve independent eightfold-coordinated M sites and a partly occupied T-cavity.
URL: http://www.minsocam.org/msa/AmMin/AmMineral.html
Subject new mineral
crystal structure

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