| Summary: | 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, 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: 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 sigma(Fo)] 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 ...
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