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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

   

Typical chemistry of pyroxenes, micas and amphiboles from some magmatic rock types

Burnaeva M.Yu. *, Krasnova N.I.**

* *FGUP BNIIOkeangeologia, St.-Petersburg, Russia;           burnaevam@mail.ru

** St.-Petersburg State University, Russia;                                           nataly_krasnova@rambler.ru

 

A diversity of a chemical composition of pyroxenes (1408 an.), micas (1776 an.) and amphiboles (735 an.) is presented in the form of RHA-data for various types of magmatic and metamorphic rocks. The full versions of these data are available at the web site: http://geology.spbu.ru/department/scientific/rha-language-method/. The typical chemistry of these minerals is defined for some of rock types.

 

Display of chemical composition of rockforming minerals pyroxenes, amphiboles and micas from rock compositions is usually made by binary or triangular diagrams, each of which gives the information on correlations only of a small number of components. Fields of mineral compositions from different rock types are often overlapping, what does not give an opportunity to carry out an unequivocal comparison of data and does not allow representing them clearly. Using of these chemical mineral properties is of great importance for diagnostics of some volcanic rocks (basalts, andesites, lamprophyres, kimberlites, lamproites), containing their phenocrysts, and to descript electromagnetic fractions of concentrates investigated by prospecting works.

The databanks on chemical composition: of 1408 clinopyroxenes (Cpx), compilated after published analyses (Dobretsov et al., 1971), with addition of our own 180 analyses from the Spitsbergen rocks; of 735 amphibole (Amf) analyses and of 1776 mica (Mica) analyses of based at materials from reference books (Minerals, 1981; Deer, et al., 1962,1963; Ushakova, 1980). Initial sets of analyses and some results of their processing using the RHA method are available at the web site: http://geology.spbu.ru/department/scientific/rha-language-method/.

For the present work, we selected only those mineral analyses the host rock names of which were fixed: ultrabasic, basic, alkaline ijolite- and nepheline-syenite series, lamproites, kimberlites, metamorphic eclogites and their xenoliths in kimberlites. The full data sets for different rock types, including acid and some metamorphic ones, are accessible in the above web site, the files: RHA-Cpx_rock groups, RHA-Amf_rock groups, RHA-mica_rock groups. The comparison of mafic rockforming minerals (MRM) compositions was carried out using program Petros-2 first of all by the rank formulas Rchem sequences of the chemical elements ordered according to decreasing of their amounts (at. %) in the analysis. One must remember, that close chemical compositions always have similar Rs. Comparison of reduced (up to 5th or 6th positions) rank formulas for Cpx, Amf and Mica show, that they actually dont differ very much within following rock groups: 1) ultrabasic: peridotites, dunites, pyroxenites, websterites; 2) basic and intermediate: gabbro, anorthosites, basalts, andesites, diorites; 3) the alkaline-ultrabasic: urtites, ijolites, melteigites; 4) intermediate alkaline: nepheline syenites; 5) lamproites; 6) eclogites; 7) kimberlites. Thus, the further comparison of MRM compositions was fulfilled between these incorporated rock groups.

The clinopyroxenes appeared to be the least changeable in composition depending on rock type, their 4 most widespread rank formulas are similar OSiMgCaAl, OSiMgCaFe, OSiCaMgAl, OSiCaMgFe for 1, 2, 5 and 6 groups (Tab. 1, in column N they are marked bold). In these rock groups the Cpx are represented by Ti-, Cr-, Fe-augite, and only in the 6th group of eclogites omphacite often occurs, which causes the presence of Na and Al at 3-5 ranks in their R. The pyroxenes of the nepheline syenites differ more clearly aegirine is characteristic for these rocks, reflecting in their most typical R: OSiNaFe or OSiFeNa.

Table 1. The reduced rank formulas for chemical composition of clinopyroxenes (Cpx) from 6 rock groups.

For amphiboles: pargasite, kaersutite, anthophyllite, hastingsite, tschermakite, occurring mainly in 1), 2) and 6) rock groups the following most typical R are characteristic: OSiMgHFe, OSiMgAlH, OSiMgAlCa. This complicates to use their chemical compositions for distinction of these rocks. It is noteworthy the frequent presence of Cr impurity (9 rank) in Amf from dunites and peridotites and Ti in Amf from pyroxenites, lherzolites, gabbro-basalts, and diorite-andesites. It is known, that amphiboles in lamproites have the typochemical composition they are represented by potassicrichterite, in R of which potassium occupies the 5-7th ranks. Amphiboles of nepheline syenites, as well as pyroxenes, show high contents of Na and Fe (3-5th ranks in their R), due to presence of arfvedsonite, eckermannite and magnesioriebeckite in these rocks.

Micas in these rock types mostly are dark coloured and are represented by phlogopite, biotite, annite and tetraferriphlogopite. Recently the method for unequivocal distinction of these dark coloured micas by their rank formulas was suggested (Krasnova et al., 2008). Difference of tetraferriphlogopite composition by RHA-method is established due to presence of rearrangements of symbols Al and K in their rank formulas in comparison with Rs of usual phlogopite (Tab. 2). Really, only by the deficit Al (correspondingly, K>Al in R) the possibility for Fe3+ to occupy the tetrahedral positions realizes. It is actually to remind, that in normal phlogopite, in contrary, Al prevails above K. According to 196 analyses of biotites the consequence of indexes Si>Al>(Fe,Mg) is typical for their Rs, while in the composition of annites (only 13 an.) Fe is dominating over Al (i.e. Si>Fe>Al). The annite-like micas arise to be characteristic mainly for the 4th group of nepheline syenites, and only 4 analyses (from 17) it is possible to attribute as biotites. In lamproites (5th group) mainly tetraferriphlogopite (18an.) occurs; two of analyses (=Phl) were excluded from this data set as xenocrystals. Phlogopite, tetraferriphlogopite and rarely biotite are found in kimberlites (group 7, 53 an.) Phlogopite is prevailing in ultrabasic rocks, and biotite is typical for the 2nd group gabbro-diorite row.

Table 2. The reduced rank formulas for chemical composition of phlogopite, tetraferriphlogopite and micas of biotite series from kimberlites, lamproites and nepheline syenites.

Thus, the mafic minerals from considered groups of rocks show some characteristic features of their chemical composition what can help their correct diagnostics. Presence of mineral composition data, even based on only microprobe analyses, allows determining the definite mineral species. It is desirable to avoid publications of minerals analyses with uncertain terms, type pyroxene, amphibole, and mica.

 

References:

Deer W.A., Howie R.A. and Zussman J. Rock-Forming Minerals. 1963. V. 2. Chain Silicates. & 1962. V. 3. Sheet Silicates. London: Longman.

Dobretsov N.L., Kochkin Yu.N., Krivenko A.P., Kutolin V.A. The rockforming pyroxenes. 1971. Moscow: Nauka Press. 120 p. (In Russian).

Krasnova N.I., Petrov T.G., Retjunina A.V. Practical aspects of the RHA method use for systematization of mica group mineral composition // Vestnik of Saint-Petersburg Univ. 2008. Ser. 7. No 2. P. 3-19 (In Russian).

Minerals. Reference book. (Chukhrov F.V., Smolyaninova N.N., eds.) 1981. V. III. No 2. Silicates with linear, three-member groups, rings and chains of silicon-oxygen tetrahedrons. & V. III. No 3. Silicates with double-chains of silicon-oxygen tetrahedrons. Moscow. Nauka Press. (In Russian).

Ushakova E.N. Biotites of magmatic rocks. 1980. Novosibirsk. Nauka Press, Siberia dept. 328 p. (In Russian).