Lobanovite, K2Na(Fe42+Mg2Na)Ti-2(Si4O12)(2)O-2(OH)(4), a new mineral of the astrophyllite supergroup and its relation to magnesioastrophyllite

摘要

Lobanovite, K2Na(Fe42+Mg2Na)Ti-2(Si4O12)(2)O-2(OH)(4), is a new mineral of the astrophyllite supergroup from Mt. Yukspor, the Khibiny alkaline massif, Kola Peninsula Russia. It has been known previously under the following names: monoclinic astrophyllite, magnesium astrophyllite, magnesiumastrophyllite and magnesioastrophyllite but has never been formally proposed and approved as a valid mineral species by the Commission on newMinerals, Nomenclature and Classification of the International Mineralogical Association. It has now been revalidated and named lobanovite after Dr. Konstantin V. Lobanov, a prominent Russian ore geologist who worked in the Kola Peninsula for more than forty years (Nomenclature voting proposal 15-B). Lobanovite has been described from pegmatitic cavities on Mt. Yukspor where it occurs as elongated bladed crystals, up to 0.04 mm wide and 0.2 mm long, with a straw yellow to orange colour. Associated minerals are shcherbakovite, lamprophyllite, delindeite, wadeite, umbite and kostylevite. Lobanovite is biaxial (-) with refractive indices (lambda= 589 nm) alpha = 1.658, beta(calc.) = 1.687, gamma = 1.710; 2V(meas).= 81.5-83 degrees. Lobanovite is monoclinic, space group C2/m, a = 5.3327(2), b= 23.1535(9), c = 10.3775(4) angstrom, beta= 99.615(1)degrees, V = 1263.3 (1)angstrom(3), Z = 2. The six strongest reflections in the powder X-ray diffraction data [d (angstrom), I, (hkl)] are: 3.38, 100, (003); 2.548, 90, (063); 10.1, 80, (001); 3.80, 60, (042,131); 3.079, 50, (132,062); 2.763, 90, ((1) over bar 71). The chemical composition of lobanovitewas determined by electron-microprobe analysis and the empirical formula (K1.97Ba0.01)(Sigma 1.98)(Na0.65Ca0.14)(Sigma 0.79)(Fe3.182+Mg2.02 Na-1.00 Mn-0.72)(Sigma 6.92)(Ti1.99Nb0.06)(Sigma 2.05)[(Si8.01Al0.06)(Sigma 8.07)O-24] O-2(OH)(4.03)F-0.19 was calculated on the basis of 30.2 (O+ OH+ F) anions, with H2O calculated from structure refinement, D-calc. = 3.161 g cm(3). In the structure of lobanovite, the main structural unit is the HOH block, which consists of one close-packed O (Octahedral) and two H (Heteropolyhedral) sheets. The M(1-4) octahedra form the O sheet and the T4O12 astrophyllite ribbons and [5]-coordinated Ti-dominant D polyhedra link through common vertices to form the H sheet. The HOH blocks repeat along [001], and K and Na atoms occur at the interstitial A and B sites. The simplified and end-member formulae of lobanovite are K2Na [(Fe2+,Mn)(4)Mg2Na]Ti-2(Si4O12)(2)O-2(OH)(4) and K2Na(Fe42+Mg2Na)Ti-2(Si4O12)(2)O-2(OH)(4), respectively.