50 New Data on Minerals. M., 2004. Vol. 39 UDC 549.742 GENETIC MINERALOGY OF THE BURBANKITE GROUP Yulia V. Belovitskaya Geological Faculty, Lomonosov Moscow State University, Moscow, Russia, [email protected] Igor V. Pekov Geological Faculty, Lomonosov Moscow State University, Moscow, Russia, [email protected] The burbankite group consists of six mineral species with general formula А3В3(СО3)5 where А = Na > Ca, REE3+, ; B = Sr, Ca, Ba, REE3+, Na: burbankite, khanneshite, calcioburbankite, remondite(Ce), remon- dite(La), and petersenite(Ce). The burbankite structural type (space group P63mc) is exclusively stable for chemical composition variations: khanneshite, calcioburbankite, remondite hexagonal analogue, and bur- bankite are isostructural and form the system of continous solid solutions. All chemical compositions (94 analyses) of the burbankite group minerals can be described within the isomorphous system with end mem- 2+ 2+ bers: (Na2Ca)М 3(CO3)5 and Na3(REE2Na)(CO3)5, where М =Sr, Ba, Ca. There are three genetic types of the burbankite mineralization: 1) in carbonatites where the minerals with the “most averaged” chemical composi- tion and increased contents of Ba and Ca are formed; 2) in alkaline hydrothermalites where the range of chem- ical compositions of the burbankitelike phases is extremely wide; 3) in pectolite metasomatites where bur- bankite is strongly REEdepleted. In carbonatites the burbankite group minerals are early phases formed under hightemperature conditions, whereas in nepheline syenite massifs they are formed during hydrother- mal stages under low temperatures, which is due to different regime of CO2. Under alkalinity decrease the bur- bankite group minerals are replaced by a whole series of secondary minerals, among which the alkalifree car- bonates of REE, Sr, Ba, and Ca prevail. 5 tables, 3 figures, 50 references. The burbankite group consists of six miner- colours: yellow, green, palebrown, pink. Fre - al species with general formula А3В3(СО3)5 quently colorless and white, less often red, where А = Na > Ca, REE3+,; B = Sr, Ca, Ba, orange and gray varieties occur. 3+ REE , Na: burbankite (Na,Ca, )3 (Sr,REE, Ba, Burbankite is a widespread mineral, where- Ca)3 (СО3)5, khanneshite (Na,Ca)3(Ba,Sr, REE,Ca)3 as other members of the group are rare. In one (СО3)5, calcioburbankite (Na,Ca,REE)3 (Ca,REE, of types of «rareearth carbonatites» (Khibiny, Sr)3 (СО3)5, remondite(Ce) Na3(Ce,Ca,Na,Sr)3 Vuoriyarvi, Gornoe Ozero etc.) burbankite and (СО3)5, remondite(La) Na3(La,Ce,Ca)3(СО3)5, its alteration products will form huge accumu- and petersenite(Ce) (Na,Ca)4(Ce,La,Sr)2 (СО3)5. lations, being the main potentially industrial The first three minerals are hexagonal (space component and easily enriched complex ore of gro up P63mc), and others are pseudohexagonal REE, Sr, and Ba. monoclinic (sp. gr. P21, g = 119.8–120.5°). In spite of a semicentennial history of In the crystal structures of hexagonal mem- research, significant number of the publica- bers of this group there are two independent tions, and extensive geography of finds, gener- cationic sites — А (Na и Са) and В (REE, Sr, Ba alizing papers on the burbankite group miner- и Са), and three types of carbonate groups with als are absent. We have attempted to system- different orientations. Tenvertex Bpolyhedra atize earlier published materials and having connected to CO3groups by vertices form the supplemented them with comparable volume layers of sixmember rings parallel (001). of new data to show the connection of chemical Eightvertex Apolyhedra form infinite zigzag composition and structural features of these columns where neighboring polyhedra are minerals with conditions of their formiation. contacted by planes (Voronkov, Shumyats ka ya, We have studied 32 samples from eight alka- 1968; Effenberger et al., 1985; Belovitskaya et al., line complexes — Khibiny, Lovozero (Kola Pe - 2000, 2001, 2002). The crystal structure of n insula), Vuoriyarvi (Northen Kareliya), Vish - remondite is quite similar to that of burbankite ne vye Gory (Southern Urals), Gornoe Ozero, (Ginderow, 1989). In the crystal structure of Mu r un (East Siberia), Mont SaintHilaire (Que - petersenite atoms of Na occupy with order two bec, Canada), and Khanneshin (Afgha nistan). Bpolyhedra out of six, which results to dou- The cation composition of the minerals bling of aparameter (Grice et al., 1994). (Tables 1–3) was studied by electronmicro- Burbankite group carbonates form hexago- probe method. All analyses including refer- nal prismatic crystals but occur more often as ence data were calculated on charge sum equal irregular grains and their aggregates. These 10.00, i.e. equivalent (CO3)5. Bsite was filled up minerals are transparent, without cleavage, to 3.00 atoms per formula unit (apfu) by atoms have vitreous up to greasy luster and light of Sr, Ba, REE, Th, K, in case of their deficiency Genetic mineralogy of the burbankite group 51 by atoms of Ca, and then atoms of Na was REEdepleted burbankite, connected to specif- added. After that the rest of Na and Ca atoms ic pectolite metasomatites of Khibiny and was placed in Asite. If the Acations sum ap - Murun massifs. In each case the minerals are peared less 3.00, the missing value was attrib- cha racterized by individual features of cation uted to vacancy according to the crystalloche - ratios (Fig. 1). Burbankite from sodabearing mi cal data (Effenberger et al., 1985; Be lov it - sedimentary Green River Formation (USA) is in skaya et al., 2000). At calculation the atomic association with the majority of the same miner- mass of Ce is conditionally taken for old analy- als as in carbonatites and alkaline hydrother- ses where the rareearth elements were deter- malites (Fitzpatrick, Pabst, 1977) and, probably, mined as a sum. The cation composition of the has lowtemperature hydrothermal origin. burbankite group minerals widely varies (Fig. Occurrences of the burbankite group minerals 1, 2). In Asites, sodium always prevails (1.3– with known chemical composition are briefly des - 3 apfu), but sometimes amounts of calcium is cribed in Table 5. They are grouped for genetic also essential (up to 1.25 apfu). The cation com- types. Localities connected to rocks enclosing car- position is more diverse in Bsites where atoms bonatites and also with products of hydrothermal of Sr, Ba, Ce, La, and Ca can dominate. activity in carbonatites are conditionally referred We make the Xray powder studies for 11 to carbonatite type. The finds made in late parage- samples (Table 4) including five species with neses of pegmatite from nepheline syenite com- different chemical composition, which crystal plex are referred to alkaline hydrothermalites. structures was refined by Rietveld method: Thus, burbankite group minerals are formed in 1) REEdepleted burbankite (an. 92), its Xray alkalicarbonate systems connected to geological diffraction pattern contains distinct doublets; objects of different types. The temperature range 2) burbankite with «typical» composition (an. these minerals crystallize in is extremely wide. 64) and nonsplit peaks on Xray spectrum; A number of massifs contain carbonatites whe- 3) khanneshite (an. 21); 4) calcioburbankite re the burbankite group minerals are the main (an. 12); 5) the mineral with chemical composi- concentrators of strontium, barium, and raree - tion of re mondite(Ce) (an. 79), but according arth elements. Here burbankite and its analogs to its Xray powder diagram identical to repre- crystallize on early carbonatite formation stages sentatives of the burbankite structural type. under high temperatures (not below 500°С). That Burbankite group minerals form complex confirmed by the signs of joint growth with essen- isomorphous system with end members: tial minerals of carbonatite rocks, the presence of 2+ REEfree phases (Na2Ca)М 3(CO3)5 where the burbankite group minerals in primary inclu- М2+ = Sr, Ba, Ca and petersenite sions, and the replacement of these minerals by Na3(REE2Na)(СО3)5, without divalent cations. products of later hydrotermal stages. All chemical compositions of the minerals are In alkaline hydrothermalites the burbankite situated in interval between these two points group minerals are the late formations forming forming extended field — Fig 1, and 2a, b. In at essentially lower temperatures. Their crys- spite of two structural transitions: from hexago- tals in cavities are frequently observed togeth- nal members to mo noclinic remondite and then to er with zeolites and hydrous soda minerals. petersenite, essential mixable intermissions in this Formation temperatures for these associations system aren’t determined. These structural transi- can be estimated as 100–250°С. tions are concerned to the second type, i.e. they The difference in time of crystallization are are realized gradually, without break of chemical first of all connected to different regime of car- bonds. bon dioxide. The excess of CO2 is present in car- bonatite formation systems, and already at early stages burbankite and its analogs appear under Occurrences and formation conditions sufficient activity of sodium. In nepheline syen- Generalizing an available material, it was ite massifs, with which the burbankitebearing possible to distinguish three main genetic types hydrothermalites are in general connected, of burbankite mineralization. Each of them is increase of potential of CO2 and, accordingly, connected to alkaline rocks. The lar gest bur- the development of carbonate mineralization bankite concentrations occur in carbonatites. take place mainly on a final stage of evolution This genetic type is studied better then others. In alkaline hydrothermalites the widest varia- tions of chemical compositions and, according- Typochemism ly, the greatest species variety are observed at and structural typomorphism relatively small amounts. We have distingu is - The wide variations of cation composition hed the third genetic type, accumulations of ma ke the burbankite group minerals very in - 52 New Data on Minerals. M., 2004. Vol. 39 a b c FIG. 1. Cation ratios in the burbankite group minerals: a — from carbonatites, b — from alkaline hydrothermalites, c — from pectolite metasomatites formative in genetic relation.