Ambiguity of nepheline syenite family rock names and how to cope with this problem
Krasnova N.I., Korolev N.M., Petrov T.G.
Saint-Petersburg State University, Saint Petersburg, Russia
nataly_krasnova@rambler.ru, nl-5183@narod.ru, tomas_petrov@rambler.ru
There are no natural borders between chemical as well as mineral compositions of “different” rocks. Therefore, the fluctuation limits both of rockforming oxides and mineral contents in one rock family are usually overlapping, and the definition of their various types causes difficulties. Besides, unequivocal identification of rock types is interfered by the fact that classifications published in reference books [1, 6] are based on different principles (mineral and/or chemical composition, structure and texture, agpaite coefficient, etc.). The contradictions connected with determination of a rock name, were always sharply shown among the alkaline series, and, in particular, among representatives of nepheline (feldspathoid) syenite group.
Task of the work is: to illustrate the uncertainty of rock names in relation to their mineral composition, and also to offer a way of describing mineral composition which will promote working out of a more precise nomenclature.
Deficiency of parallel chemical and mineral content analyses of alkaline rocks has compelled to use the data of their normative composition, calculated by means of Norm-4 program applied by many geologists in the world. Here we do not discuss the problem of conformity of the mineral composition received by CIPW method to real rock composition since normative calculations in petrographic practice are standard. It is sufficient that the use of similar chemical analyses leads to receiving of similar normative compositions, irrespective of the method by which the latter were calculated.
Normative compositions for 418 chemical analyses of many alkaline rock varieties taken from 6 published and 2 own sources were calculated using Norm-4program. The results are presented in volume % (Vol. %), that corresponds mostly to visual definition of a rock. RHA-Min comp_alkaline rocks.rar file on the web-site http://geology.pu.ru/index.php?mod=mod1_6_2_4&nam=¦х=юф%20RHA&menu=6&smenu=46 displays the full version of data.
Classifications of chemical and mineral composition were made using rank-entropy RHA-method [2]. Here R – the rank formula – is a succession of components in the system ranged according to decrease of their contents in the analysis. Two other parameters – H and A – are integral characteristics of ranged components distribution. The method is described in more detail in [3, 5, 7]. As a result, we receive a set of rank formulas (in abbreviated form: R-Min) – sequences of minerals e.g. of AbNeOrt… type that corresponds to a decrease of volume ratios (percents) of minerals in rock Ab> Ne> Ort>….
Rank formula sets are ordered according to a certain “alphabet”. The sequence of these minerals received in their chemical R-classification (R-Min-Catalogue-2007.rar file on the same web-site) serves as an “alphabet”.
Obtained data are shown as rank formulas of mineral volume % contents reduced to the 3rd position. These data illustrate a well-known domination of nepheline (Ne), orthoclase or microcline (Ort) and alkaline plagioclase (Ab) complexes in composition of main nepheline syenite types. Similarity of many rank formulas for the whole syenite family is crucially important for our work. So, formulas AbNeOrt and AbOrtNe are present in groups of foyaites, khibinites, mariupolites, miaskites and lujavrites; formula OrtNeAb is common in groups of foyaites, khibinites, rischorrites and juvites (data on the last two are available only in the full table on the web-site); OrtAbNe formula – in groups of foyaites, khibinites, miaskites and lujavrites. Thus, the above-stated data evidently confirm the inconsistency of classifications suggested in [6], and show that rigid criteria allowing unequivocal definition of a rock type are not fixed to them in nepheline syenites family.
For resolving the next problem, the authors offer to use the following method BEFORE the establishment of precise criteria of division on rock types during identification of varieties in nepheline syenite group. It is possible to add to the term «nepheline syenite» the rank formula of mineral composition, reduced to three (or four) positions, receiving the rock name e.g. «nepheline syenite OrtAbNe». Such names can be formed on the basis of quantitative calculations of their real mineral composition. First, it will allow considering really present minerals, instead of virtual components, as K2SiO3, Na2CO3, corundum, etc. received sometimes during norms calculation. Secondly, it will provide the account of minerals with volatile components, which include micas, amphibole, cancrinite, etc. occurring in considered rocks. As is known, the presence of these minerals does not come to light by using Norm-4 program.
It will be possible to work out a rational nomenclature for various rock groups of this family after a critical revision of such representative RHAmin collections from different regions. The values of complexity (H) and purity (A) of composition, showing moniminerality degree of a rock can be used for specification of the description in the form of rank formulas.
RHA-method allows creating a uniform, unequivocal classification of mineral compositions not only of nepheline syenite group, but also for the whole rock variety. Such classification is open for addition of new types, which were unknown before. The opportunity to create an unequivocal rock classification based on their mineral composition using the mineral composition rank formulas was earlier realized on an example of phoscorites and carbonatites [4, 7] – rocks which did not have a standard nomenclature. Besides, success of the offered method application is shown on an example of well-studied rocks: granites (77 analyses), diorites (59), and gabbros (33 analyses). These results are accessible on separate pages of RHA-Min comp_alkaline rocks.rar file. Increase of description detaility (rank formula length) will allow taking into consideration the peculiarities of rock mineral composition significant for particular cases. Unambiguity of alphabetic ordering of RHA-descriptions of compositions allows to store numerous data sets and to reveal groupings of similar rocks by their composition, to estimate completeness of data for various groups, to reveal mistakes of different types. Presence of rather representative arrays of rock types stipulates the opportunity of their identification by real modal composition.
Table. Reduced rank formulas of mineral composition (Rmin) for different representatives of nepheline syenite family
(N – quantity of analyses corresponding to given rank formula).
Abbreviations: Ab – albite; Aeg - aegirine; Crn - corundum; Di - diopside; Ne - nepheline; Ol - olivine; Ort - orthoclase.
References:
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