KALIFERSITE, A NEW ALKALINE SILICATE FROM KOLA-PENINSULA (RUSSIA) BASED ON A PALYGORSKITE-SEPIOLITE POLYSOMATIC SERIES

Kalifersite, (K, Na)(5)(Fe3+)(7)[Si20O50](OH)(6). 12H(2)O, is a new [0 01] fibrous silicate discovered in a drill core of a hydrothermally al tered pegmatite in the urtite series at Mt. Kukisvumchorr (Kola Penins ula). The new mineral formed by crystallization from residual peralkal ine liquids during the hydrothermal stage of the pegmatitic process. P ink-brownish; good {100} and {010} cleavages; D(meas) = 2.37(2) g/cm(3 ). Biaxal (+); alpha = 1.523(2), beta = 1.525(2), gamma = 1.550(2), 2V (meas) = 30(2)degrees, 2V(calc) = 32 degrees; optically pseudo-orthorh ombic, O.P. (010); slightly pleochroic. SAED patterns show triclinic s ymmetry and streaks; a = 14.86(4), b = 20.54(4), c = 5.29(2) Angstrom, alpha = 95.6(3), beta = 92.3(3), gamma = 94.4(3)degrees have been obt ained from least-squares refinement of powder diffraction data; s.g. P (1) over bar, Z = 1. Comparison with sepiolite {Mg-8[Si12O30](OH)(4). 12H(2)O; a = 13.40, b = 26.80, c = 5.28 Angstrom, s.g. Pncn, Z = 2} a nd palygorskite {Mg-5[Si8O20](OH)(2). 8H(2)O; a = 13.27, b = 17.868, c = 5.279 Angstrom, beta = 107.38 degrees, s.g. C2/m, Z = 2} allowed to obtain for kalifersite a structural model which explains its characte ristics. On the basis of P = y),square(z),][Si8O20](OH)(2){(A(n+))((2z '-y')/n). m'H2O} (palygorskite module) and S = +)(y)square(z)][Si12O30 ](OH)(4){(A(n+))((2z-y)/n). mH(2)O} (sepiolite module), kalifersite is the P1S1 member (with y + y' = 7, z + z' = 6 and m + m' = 12) of a Pp Ss polysomatic series named palysepiole series. In kalifersite, the al kali cations A lie in octahedra which connect the Y-octahedra belongin g to the palygorskite/sepiolite framework. This feature and polysomati c aspects are discussed by comparison with biopyriboles, raite and, in general, 2:1 layer silicates.