Carraraite and zaccagnaite, two new minerals from the Carrara marble quarries: their chemical compositions, physical properties, and structural features

Abstract

Two new mineral species, carraraite and zaccagnaite, were found in cavities in calcite veins in marble quarries of the Carrara basin (Apuan Alps, Italy). Carraraite, Ca 3 Ge(OH) 6 (SO 4 ) 1.08 (CO 3 ) 0.92 ⋅12H 2 O, occurs as submillimetric crystals, tabular on {001}. The cell dimensions are a = 11.056 (3), c = 10.629 (6) Å, and the space group is P63/m. Carraraite is optically uniaxial (-), ω = 1.509, ε = 1.479. The strongest lines of the X-ray diffraction pattern are at d-spacings (Å): 9.57 (vs) (100), 5.53 (s) (110), 3.83 (s) (112), 3.56 (ms) (202), 2.74 (ms) (302). Carraraite is a new member of the ettringite-thaumasite group, which is characterized by columns of composition [Ca 3 Ge(OH) 6 ·12H 2 O] 4+ running along c and interconnected through hydrogen bonding to (SO 4 ) 2- and (CO 3 ) 2- groups. Zaccagnaite, Zn 4 Al 2 (OH) 12 (CO 3 )⋅3H 2 O, occurs as minute hexagonal crystals, elongated parallel to [001]. The cell dimensions are a = 3.0725 (3), c = 15.114 (4) Å and the space group is P63/mmc. The crystals are always covered by a thin crust of fraipontite. The strongest lines of the X-ray diffraction pattern are at d-spacings (Å): 7.51 (vs) (002), 3.794 (m) (004), 1.542 (ms) (108), 1.539 (ms) (110). Zaccagnaite is a new member of the hydrotalcite-manasseite family; its structure is characterized by a regular alternation of brucite-like layers with composition (Zn 2/3 Al 1/3 )(OH) 2 and an interlayer composed of carbonate groups and water molecules.