2013

Presentation of mineral composition of non-completely crystalline rocks

using the RHA information language

Burnaeva M.J. *& Krasnova N.I. **

FGUP “VNIIOkeangeologia”, St. Petersburg, Russia

** St. Petersburg State University, Russia

nataly_krasnova@rambler.ru

Up to now, there are difficulties in diagnostics of porphyritic rocks with partly crystalline matrix. Earlier, the principles and results of ordering of minerals and holocrystalline rocks, both in chemical and modal and normative mineral (for rocks) composition were stated with use of RHA method [1]. This method allows to diagnose such rocks more definitely and discriminatingly. R is a sequence of components of system on reduction of their content in this system. As a result of such ordering, rank formulas - sequences of minerals of OlivBiotDipsPyrt type are received, that corresponds to volume fractions (Vol. %) of minerals in Oliv> Biot> Dips> Pyrt phenocryst rocks. Sets of rank formulas are ordered under a certain "alphabet". A sequence of these minerals received in their chemical R-classification of all minerals (R-Min-Catalogue-2012.rar file under the same Internet address) serves as an “alphabet”. Phenocrysts, as a rule, are specified either optically, or by exact methods on their chemical composition. Mineralogical diagnostics of matrix using optical methods is often impossible due to the small size of individuals. Using Hitachi TM 3000 device (Swiffed 3000 – a microprobe microscope), in magmatic hypabyssal rocks of the Svalbard archipelago we defined composition of phenocrysts in the form of individual points, and in the form of areas - composition of the matrix (figure). The proposed classification is based on the mineral composition of porphyritic phenocrysts and norm mineral composition of the matrix (table). Rocks are ordered first of all on phenocrysts composition, and in case of their coincidence - on norm composition of the ground mass. Our results were compared to the rank formulas received for 92 mineral composition of porphyritic rocks listed in [2]. Our rocks appeared to be the closest in R to composition of picrites. In the above paper, unfortunately, plagioclase diagnostics is not specified in rocks, that narrows the completeness of results representation. Demonstrativeness of the received results and unambiguous ordering of mineral compositions of rocks offered by the method can facilitate identification of petrographically difficult for determination of non-completely crystalline rocks.

For abbreviators of minerals see [1]. The first line in the table corresponds to the highlighted area 4 at the micrograph.

1. http://geology.spbu.ru/department/scientific/rha-language-method/
2. Bogatikov O.A., Kosareva L.P., Sharkov E.V. (1987) Average chemical composition of magmatic rocks. Moscow: Nedra Press. 153 p.