Caysichite-(Y) from the Ploskaya Mountain (Kola Peninsula, Russia): Crystal-Structure Refinement and the Chemical Formula

Abstract

The crystal structure of caysichite-(Y) from the Ploskaya Mt (Kola Peninsula, Russia) has been refined to R₁ = 0.051 for 4472 unique observed reflections. The mineral is orthorhombic, Ccm2₁, a = 13.2693(3), b = 13.9455(4), c = 9.7384(2) Å, V = 1802.06(8) ų, Z = 4. There are two M sites predominantly occupied by Y, but also including Ca and other rare earth elements (REEs). Both M sites are coordinated by eight O atoms to form distorted bicapped trigonal prisms. The crystal structure is based upon a three-dimensional framework formed by columns of MO₈ polyhedra and (CO₃) groups and double-crankshaft chains of SiO₄ tetrahedra running parallel to the c-axis. The topology of linkage of MO₈ polyhedra understood in terms of the M–M links shorter than 5 Å corresponds to the M network with the paracelsian (pcl) topology. The channels in the network are occupied by double-crankshaft Si chains and H₂O groups. The new general chemical formula of a caysichite-(Y)-type mineral can be written as [Y_2+2x−y′Ca_{2−3x−y″}☐_x+y′+y″][Si₄O₁₀](HCO₃)_3y′+2y″(CO₃)_{3−3y′−2y″}·(4−z)H₂O, where z ~ 0.2; x ≤ 2/3; y′ ≤ 2/3; y″ ≤ 1; 3y′+2y″ ≤ 2. This general formula allows for several end-member formulas according to different x, y′ and y″ values: (Y₂Ca₂)[Si₄O₁₀](CO₃)₃·4H₂O (x = y′ = y″ = z = 0), (Y₂Ca☐)[Si₄O₁₀](HCO₃)₂(CO₃)·4H₂O (x = y′ = z = 0; y″ = 1), (Y_10/3☐_2/3)[Si₄O₁₀](CO₃)₃·4H₂O (y′ = y″ = z = 0; x = 2/3), Ca₂Y_4/3☐_2/3)[Si₄O₁₀](HCO₃)₂(CO₃)·4H₂O (x = y″ = z = 0; y′ = 2/3). The samples studied in this work have the compositions (REE_2.05Ca_1.87☐_0.18)[Si₄O₁₀](HCO₃)_0.11(CO₃)_2.89·3.8H₂O (x = 0.025, y′ = 0, y″ = 0.055) and (REE_2.25Ca_1.52☐_0.23)[Si₄O₁₀](HCO₃)_0.21(CO₃)_2.79·3.8H₂O (x = 0.125, y′ = 0, y″ = 0.115). The end-member formula most close to these compositions is (Y₂Ca₂)[Si₄O₁₀](CO₃)₃·4H₂O, which is different from the formula (Ca,Yb,Er)₄Y₄(Si₈O₂₀)(CO₃)₆(OH)·7H₂O currently adopted by the International Mineralogical Association but is generally identical to the formula (Y,Ca)₄Si₄O₁₀(CO₃)₃·4H₂O proposed in the original study of the mineral. In order to resolve the problem of the caysichite-(Y) formula, additional studies of materials from different localities (and, especially, one from the holotype locality) are needed.