Caysichite-(Y) Ca4(Y,REE)4(Si8O20)(CO3)6·8H2O: Endmember Composition and Structure Established from New Occurrences Across Different Geologic Settings

Abstract

Abstract The heavy rare earth element silicate-carbonate mineral caysichite-(Y) [Ca4(Y,REE)4(Si8O20)(CO3)6·8H2O] was discovered in the alkaline granite of the Golden Horn Batholith, near Washington Pass, Okanogan County, Washington, USA, as sub-millimeter euhedral orthorhombic pale white crystals in a single miarolitic cavity in biotite granite. Based on comparative electron microprobe, single-crystal X-ray diffraction, and Raman analyses of caysichite-(Y) from both the Golden Horn and Mount Malosa (Malawi) localities, the ideal endmember composition and space group Pnmn with orthorhombic unit cell parameters a = 13.2577(5) Å, b = 13.9816(5) Å, and c = 9.7440(4) Å was determined. This structure and range of composition is distinct from caysichite-(Y) from the type locality Evans-Lou (Canada), where re-investigation confirms the space group Ccm21, with high REE ΣREE (0.7–1.0) apfu and Al3+(0.02–0.23) apfu in agreement with the original work of Mellini & Merlino (1978) and Hogarth et al. (1974), respectively. In contrast, Golden Horn and Mount Malosa caysichite-(Y), with an absence of Al3+ and ΣREE (0.5–0.8) apfu, exhibit a euhedral crystal habit with distinct Ca-Y site ordering, establishing the ideal endmember composition of Ca4(Y,REE)4(Si8O20)(CO3)6·8H2O. The Evans-Lou composition, close to (Ca,REE)3(Y,REE)5(Si8O20)(CO3)6(OH)·7H2O, derives from the two coupled substitutions Ca2+ + H2O ↔ REE3++ OH− and Si4+ + Ca2+↔ REE3++ Al3+. The substitutions together with the Ca-Y site disorder suggest rapid crystallization from a secondary hydrothermal solution as fine grained acicular crusts, whereas the well-defined Golden Horn and Mount Malosa caysichite-(Y) crystals appear to have derived from late-stage primary fluids.