MEGAKALSILITE, A NEW POLYMORPH OF KAlSiO4 FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: MINERAL DESCRIPTION AND CRYSTAL STRUCTURE
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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 ϫ 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:
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The paper studies accessory Cr-spinels from deep drill holes crossing the Nude-Poaz massif, which is a part of the Monchegorsk mafic-ultramafic layered complex (2.5 Ga, Kola Peninsula, Russia). Cr-spinels occur as two morphological types that differ in their chemical composition, i.e., Cr-spinels of the first type are more aluminous, while Cr-spinels of the second type are more ferruginous and titaniferous. Cr-spinels of the Nude-Poaz massif are characterized by a Fe-Ti trend known for layered intrusions in the world. Cr-spinels of the Nude-Poaz massif quite clearly differ in composition from chromites of the Sopcheozero deposit: they are more ferruginous and less chromous. The specific composition of Cr-spinels in rocks of the Nude-Poaz massif can be correlated with the sequence of the magmatic phases intrusion.
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