16. Mineralogy o Alkaline Rocks MICROHETEROGENEITY AND STRUCTURE ORDERING IN RHABDOPHANE GROUP MINERALS IN PSEUDOMORPHS APATITE GROUP MINERALS: IR SPECTROSCOPIC INVESTI­ AFTER STEENSTRUPINE-(Ce) FROM LOVOZERO ALKALINE GATION COMPLEX, KOLA PENINSULA. Chukanov N.V. (Institute of Chemical Physics in Chernogolovka, Russia, Ekimenkova I.A., Pekov I. V. and Kononkova N .N. (Dept. of Geology, [email protected]) and Pekov l.V. (Dept. of Geology, Moscow State Moscow State University, [email protected]) University) The rhabdophane group minerals are typical product of low-alkaline Two series of apatite group minerals have been studied: RE-bearing and steenstrupine-(Ce) alteration. OH-enriched. In many peg111atites of the Lovozero complex there are pseudomorphs One can distinguish two subgroups among RE-bearing apatite-like after belovite-(Ce) and vitusite-(Ce) composed of silicon-free minerals: RE-dominant members - belovite·(Ce), belovite-{La) (Pekov rhabdophane-(Ce). Whereas all examined rhabdophane group minerals e.a., 1996), and deloneite-(Ce) (Khomyakov e.a., 1996) and minerals with composing pseudomorphs after steenstrupine from the same pegmatites lower REE content ... fluorcaphite (Khomyakov e.a., 1997) and contain admixture of silicon prevailing over phosphorus in the samples RE-varieties of fluorapatite. According to the data of IR spectral contour from the Natrolite Stock pegmatite. Its formula is analysis, the second subgroup includes several types of "RE-apatites." 1. (Ce0.• Lao.iC8o.1sTho.1sNdo. 1)1.o(Si0.Jl0.s)1.104*H20, therefore it is not correct to ''Saamite" from Lovozero alkaline massif, Kola peninsula (N~O 0.8, SrO name this phase as rhabdophane proper. Thus we propose the te11n "sili­ 9.5, RE:EiOJ 4.1 % wt.), is fluorapatite containing microclusters (<0.n mm) ceous rhabdophane" like (but not identical) to that of E.I.Semenov's (1959) of belovite-(Ce). 2. Sr-poor ''NaRE-apatite" from Ilimaussaq alkaline "Th-Si rhabdophane". This mineral is clearly shown by stoichiometry of complex, South Greenland (Na20 3.1, SrO<O.l, RE~03 16.7%) (Roensbo, this compound, X-ray powder data, and IR spectra to be a Si-dominant 1989), is fluorapatite containing clusters with belovite-like structure, RE-member of the rhabdophane group. perhaps "calciobelovite," CaJNaREE(P04)JF. 3. Fluorcaphite from An interesting case of Ce separation from other lanthanoids was Khibiny alkaline massif, Kola peninsula (Na20 1.6· l .9, SrO 19-21, discovered in alterated steenstrupine-(Ce) from weathered ussingite RE~03 9 .3-13 .5%), seems to be homogeneous. pegmatite at the Kamasurt Mt. Oxides of Ce4+ and Mn4+ yield aggregate, Splitting of the band of 0-H·stretching vibrations (the region 3500-3600 occurring as fine grained (less than 0.1 mm) heterogeneous weak­ cm-1) in IR-spectra of OH-bearing apatite-like minerals is appeared when crystalline mass enriched in Ce (about 30% Ce02, 6% Lai03, 2,5% Nd20 3). OH-group content exceeds 1 per unit cell. This effect was observed for All other lanthanoids were concentrated in Ce-poor rhabdophane-(La) hydroxylapatite, hydroxylellestadite, and johnbaumite. This (OH)-band with composition CL8o.s9Ndo. 17Tho.14Cao.07Ce0.03Y0.0J 1.02(P0.93Si0.06) 0.9904*H20. splittering can be explained by existence of non.equivalent positions of It consists of monomineralic balls about 0.05 mm, which consist of tiny OH-groups in apatite·like structures. It is very likely that symmetry long.prismatic crystals. Diagnostics of this phase is confirmed by x... ray lowering for some OH-rich apatite.like minerals is the reason of such powder data and IR-spectra. This fractionation of REE is caused by splitting. oxidation of Ce3+ to Ce4+ under hypergene alteration of steenstrupine. This is the first find of rhabdophane-(La) in the Khlbiny-Lovozero alkaline complex. GENTHELVITE FROM ITAPITANGUI, SAO PAULO, BRASIL RECENT CONTRIBUTINOS TO THE MINERALOGY OF POCOS DE CALDAS ALKALINE MASSIF, MINAS GERAIS, BRAZIL Coutinho J.M.V~ and Oliveira M.C.B. (Inst. Pesq. Teen. S.P. IPTISP), Atencio D. and Carvalho F.M.S. (JG-USP) Matioli P.A., Atencio D., Tsugawa J.K. (Inst. Geociencias, Univ. Sao Paulo, [email protected]) Genthelvite, Be3Zn4(Si04) 3S, cubic, is a rare mineral of the helvite group, which also includes helvite Be3Mn4(Si04) 3S and danalite, The Mesozoic Po~os de Caldas alkaline complex is circular-shaped with Be3Fe4(Si04)JS. Genthelvite forms solid solution with danalite, but not a mean diameter of about 33 km. It comprises a suite of alkaline volcanic with helvite (Dunn, 197 6). Natural occurrences of genthelvite are and plutonic rocks (mainly phonolites and nepheline syenites). Recent restricted to highly fractionated alkaline to peralkaline granites and contributions to its mineralogy includes gaidonnayite, barium-free syenites or to their associated pegmatites, and also in greisens and skams. burbankite, hilairite, taeniolite, narsarsukite, quartz, tuppersuatsiaite and The extremely high "chalcophilicity" of Zn explains, in part, why some unidentified minerals associated to hainite. Gaidonnayite and genthelvite is such a rare mineral (Burt, 1988). The first Brazilian occur­ barium-free burbankite occur as a latest interstitial filling mineral in the rence of genthelvite was first described in 1988. A new occurrence of nepheline sienite from Pedreira da Prefeitura ("City Hall Quany"), P~os genthelvite in Brazil is here described from a peralkaline granitic body, in de Caldas, Minas Gerais, Brazil. Associated minerals in the fracture are ltapitangui, southern Sao Paulo state. Only two millimetric genthelvite hilairite, calcite, fluorite, natrolite, analcite, pyrite, manganoan ilmenite, crystals were found, in riebeckite- mesopertite· quartz granite. The strontianite, ankerite, kutnahorite, and chamosite. Quartz, aegirine, fluorite macroscopic color is intense pink, similar to typical rhodonite and rhodo· and pectolite were recorded in vugs in the nepheline sienite from Pedreira chrosite, but with yellowish- brown spots of intemperic manganese oxide. da Prefeitura. Narsarsukite, aegirine, quartz, riebeckite and taeniolite were The luster is vitreous and the hardness is between 6 and 7. One crystal recorded in association to potassic feldspar in a rock from the Bortolan displayed a striated face, possibly tetrahedral, and another vicinal face with quany. The vug filling crystals of hainite of the subvolcanic phonolite intense luster. Some weak reflections seem to indicate a poor octahedral from Bortolan quarry are associated to some unidentified minerals. cleavage. The tetrahedral habit may also be deduced from the triangular Mineral (A), displaying rectangular contours and maximum dimensions of shapes seen in thin section. 8 x 0.2 µm develops along fracture surfaces of h'ainite crystals. EDS Refractive index measured with checked Cargille immersion liquids in analyses indicated a Sr, Ce, Ca, La and Na silicate-phosphate, with F and white light, by the method of the Becke line was 1. 741 ( 1). The dete1111i­ Cl. A second unidentified mineral (B), grows around the same hainite nation of specific gravity was impeded by the small crystal dimensions grain. It is anhedral, its area measures about 10 x 10 µm, and is a Ba, Na, and association with manganese oxide. The best obtained value was 3.55. Nb, Ti and Ca silicate. In another sample, hainite is associated to EDS analysis showed the expected presence of S, Si, Zn, Fe and Mn. manganoan pectolite, one other mineral similar to tuperssuatsiaite and a Secondary electron images exhibited homogeneous distribution of Zn and third unidentified mineral (C). The latter is a Ti, Na, Mn, Fe, Ca, Kand Al Fe with maximum concentration of Zn. The cell parameter a=8. l 3 l (2), silicate with rectangular contours and maximum dimensions of 120 x 15 calculated from X-ray powder diffraction data, together with the refractive µm. index was used in the diagrams of Vlasov to estimate the approximate composition: genthelvite 78%, danalite 14%, helvite 14%. • Al 11 17TH GENERAL MEETING .