MEGAKALSILITE, A NEW POLYMORPH OF KAISIO4 FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: MINERAL DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Megakalsilite, KAlSiO4, hexagonal, a 18.1111(8), c 8.4619(4) Å, V 2403.7(2) Å3, c:a = 0.4672, space group P63, Z = 24, R = 0.039 (for 3255 observed reflections collected with a four-circle X-ray diffractometer), is a new mineral species from Mount Koashva, Khibina alkaline massif, Kola Peninsula, Russia. It is of pegmatitic origin. Associated minerals are K-feldspar, sodalite, cancrinite, natrolite, pectolite, aegirine, natrite, nacaphite, vitusite, fluorcaphite, belovite, umbite, lemmleinite-K, lomonosovite, lovozerite, phlogopite, sphalerite, and galena. The mineral was found in only one hand specimen of pegmatite rock as a corroded, irregularly shaped grain 2 * 3 mm across, intergrown with cancrinite, sodalite, and natrite. It is transparent, colorless with a white streak, a vitreous luster, and fluoresces pale whitish green under ultraviolet light. Megakalsilite has a Mohs hardness of 6, is brittle with a conchoidal fracture, and has no cleavage. Dmeas is 2.58(2) g/cm3 , Dcalc is 2.62 g/cm3 . Megakalsilite is uniaxial negative, non-pleochroic, ω 1.538(1), ε 1.531(1). The strongest five reflections in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 3.091(100)(222), 2.612(70)(060), 1.240(60)(4.10.1,066,583), 3.18(50)(141), and 1.674(50)(173). An electron-microprobe analysis gives: K2O 29.73, Na2O 0.02, FeO 0.04, Al2O3 32.38, SiO2 37.96, TiO2 0.01, sum 100.14 wt.%. The corresponding empirical formula is K0.997Na0.001Fe0.001Al1.003Si0.998O4 (based on O = 4), ideally KAlSiO4. The name megakalsilite is derived from the Greek (great) and kalsilite, in allusion to the fact that megakalsilite shares the same chemical formula with kalsilite, but its unit cell is 12 times larger than that of kalsilite. The crystal structure of megakalsilite, KAlSiO4, has been solved by direct methods and refined to an R1 index of 3.82% based on 3255 observed [Fo > 4 (Fo)] unique reflections measured with MoKα radiation on a conventional four-circle diffractometer. There are four Si sites, four Al sites, and six K sites in the structure. Megakalsilite is a tectosilicate with a framework consisting of six-membered rings of regularly alternating Si- and Al-centered tetrahedra; [9]- and [10]-coordinated K atoms are located in the large cavities. The six-membered rings of tetrahedra are present in two different conformations based on the following sequences of up (U) and down (D) tetrahedra: UDUDUD and UUU. The sequence of these two different rings along x (y) results in the unit cell parameter of 18.1111 Å. UDUDUD rings are characteristic for the high tridymite topology. UUUΔ rings have been reported for KAlSiO4–O1. Crystal structures of megakalsilite and the synthetic aluminogermanate KAlGeO4 have the same framework topology.