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

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) Å3, 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 [Y2+2x−y′Ca2−3x−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), (Y10/3☐2/3)[Si4O10](CO3)3·4H2O (y′ = y″ = z = 0; x = 2/3), Ca2Y4/3☐2/3)[Si4O10](HCO3)2(CO3)·4H2O (x = y″ = z = 0; y′ = 2/3). The samples studied in this work have the compositions (REE2.05Ca1.87☐0.18)[Si4O10](HCO3)0.11(CO3)2.89·3.8H2O (x = 0.025, y′ = 0, y″ = 0.055) and (REE2.25Ca1.52☐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.