A new occurrence of alkaline magmatism on the Kola Peninsula: An agpaitic dyke in the Kandalaksha region
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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.
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在北方 Qinling 早古生代的 granitoids 标明日期的锆石 U-Pb 产出 500， 452 和 420 妈的三座年龄山峰。他们能时间地被相关与对在 ca 的超离频压力的变态高压。500 妈，后退在 ca 的 granulite 外形变态。在 ca 的 450 妈和角闪岩外形变态。420 妈分别地。花岗石的 magmatism 的第一个事件被认为源于大陆人碰撞，而 magmatism 的第二和第三个事件被归因于外壳的高举。从高压、超离频压力的变形岩石与地区性的地质的背景和新结果结合了， ca。500 妈 magmatism 作为包括北方中国 Craton 的南部的边缘的在在南方 Qinling microcontinent 和北方 Qinling 带之间的 accretionary 楔的沉积岩石的部分融化的结果被解释。ca。450 妈集中的 magmatism 被归功于到脱水融化深深地以响应平板 breakoff 的变厚的条件的 subducted 大陆人外壳，和在 ca 的最后的 magmatism。420 妈作为到扩展的从收缩的在构造转变期间的部分融化的产品被解释。
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Many geologists have been interested in the Okinawa Trough due to its unique tectonic environment.The magmatism is regarded as one of the key questions in the Okinawa Trough.The advances associated with the magmatism in the trough have been overviewed,including the studies about the characters of the magmatic sources,the processes of the magma melting and evolution and the regularities of melting.Based on these,the difficult questions on this field have been presented and a new idea that using the method of U-series disequilibria to study the magmatism in Okinawa Trough has been brought forward.Meanwhile,some research directions about the study of magmatism in the Okinawa Trough should be paid much attention to in the future,e.g.① the influence of the Philippine subduction slab on the magmatism processes,②the implications of the subducting sediment for the magma,③the controlling factors of the magma melting,④ the link between the magmatism and the seafloor hydrothermal activity.
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