961 The Canadian Mineralogist Vol. 40, pp. 961-970 (2002) MEGAKALSILITE, A NEW POLYMORPH OF KAlSiO 4 FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: MINERAL DESCRIPTION AND CRYSTAL STRUCTURE ALEXANDER P. KHOMYAKOV Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, Moscow 121357, Russia GENNADII N. NECHELYUSTOV Russian Institute of Mineral Resources, Moscow 109017, Russia ELENA SOKOLOVA Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ELENA BONACCORSI, STEFANO MERLINO AND MARCO PASERO Dipartimento di Scienze della Terra, Università di Pisa, I–56126 Pisa, Italy ABSTRACT Megakalsilite, KAlSiO 4 , hexagonal, a 18.1111(8), c 8.4619(4) Å, V 2403.7(2) Å 3 , c:a = 0.4672, space group P6 3 , 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. D meas is 2.58(2) g/cm 3 , D calc is 2.62 g/cm 3 . Megakalsilite is uniaxial negative, non-pleochroic,  1.538(1),  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: K 2 O 29.73, Na 2 O 0.02, FeO 0.04, Al 2 O 3 32.38, SiO 2 37.96, TiO 2 0.01, sum 100.14 wt.%. The corresponding empirical formula is K 0.997 Na 0.001 Fe 0.001 Al 1.003 Si 0.998 O 4 (based on O = 4), ideally KAlSiO 4 . 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, KAlSiO 4 , has been solved by direct methods and refined to an R 1 index of 3.82% based on 3255 observed [F o > 4(F o )] unique reflections measured with MoK 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. UUU rings have been reported for KAlSiO 4 –O1. Crystal structures of megakalsilite and the synthetic aluminogermanate KAlGeO 4 have the same framework topology. Keywords: megakalsilite, new mineral species, polymorph, crystal structure, Khibina, Kola Peninsula, Russia. SOMMAIRE La mégakalsilite, KAlSiO 4 , hexagonale, a 18.1111(8), c 8.4619(4) Å, V 2403.7(2) Å 3 , c:a = 0.4672, groupe spatial P6 3 , Z = 24, R = 0.039 (pour 3255 réflexions observées, prélevées avec un diffractomètre à quatre cercles), est une nouvelle espèce minérale provenant du mont Koashva, complexe alcalin de Khibina, péninsule de Kola, en Russie. Elle a une origine pegmatitique. Lui sont associés feldspath potassique, sodalite, cancrinite, natrolite, pectolite, aegyrine, natrite, nacaphite, vitusite, fluorcaphite, § E-mail address: sokolova@ms.umanitoba.ca