Linking structure and chemistry in the schorl-dravite series

The crystal structure of a Th-U-rich, Y-poor hellandite-(Ce) occurring in a volcanic ejectum from Capranica (Vico volcanic complex, Latium, Italy) was refined to an R index of 1.5% for 2226 ob­served reflections [I > 3 σ(I)]. Hellandite from Capranica is monoclinic P2/a with a = 19.068 (8), b = 4.745 (2), c = 10.289 (3) Å, p = 111.18 (3)°. The extra-framework (distorted) tetrahedral cavity, usually occupied by H in hellandite, is partially (35-40%) occupied by Be and/or Li. When this is the case, the [B 4 Si 4 O 22 ] tetrahedral chains of hellandite are locally cross-linked to form a sheet, which is built up by the same types of rings as in semenovite but in a different arrangement. The structure refinement and the electron- and ion-microprobe analyses provide constraints on hellandite-group crystal chemistry. The presence of [4] (Be, Li) at the Be site is coupled with the substitution of F and/ or O for OH at the 05 site; the amount of extra-framework cations thus constrains the OH content in the general formula. X-ray data exclude the presence of other OH sites in the structure, in agreement with direct H determination by ion probe (1.02 H apfu + 0.98 F apfu = 2.0 apfu of monovalent anions at O5 vs. 4.5-8 apfu proposed previously). The complete chemical analysis also eliminates the need for a vacancy at the REE sites. The general formula of hellandite-group minerals is thus (Ca,REE) 8 (Th,U,Y,REE) 4 (Ti,Fe 3+ ,Al)2(O,F,OH x ) 4 (Be,Li) 4-x Si 8 B 8 O 44 , where electroneutrality is assured by the appropriate mixing of heterovalent substituents at the M sites.