Abstract
A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).
The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe2+ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Aliv to balance the effect of increasing (Fe3++Ti4++Alvi+Cr3+)M1.
The Fe2+/Fe3+ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe2+ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe2+ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.
In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.
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Aurisicchio C, Brotzu P, Morbidelli L, Piccirillo EM, Traversa G (1983) Basanite to peralkaline phonolite suite: Quantitative crystal fractionation model (Nyambeni Range, East Kenya Plateau). Neues Jahrb Mineral Abh 148:113–140
Brotzu P, Morbidelli L, Piccirillo EM, Traversa G (1983) The basanite to peralkaline phonolite suite of the plioquaternary Nyambeni multicentre volcanic range (East Kenya Plateau). Neues Jahrb Mineral Abh 147:253–280
Burnham CW, Clark JR, Papike JJ, Prewitt CT (1967) A proposed crystallographic nomenclature for clinopyroxene structures. Z Kristallogr 125:109–119
Clark JR, Papike JJ (1968) Crystal chemical characterisation of omphacites. Am Mineral 53:840–868
Colby JW (1972) MAGIC IV, a computer program for quantitative Electron Microprobe Analysis. Bell Telephone Laboratories, Inc, Allentown, Pennsylvania
Cundari A (1979) Petrogenesis of leucite-bearing lavas in the Roman Volcanic Region, Italy. The Sabatini lavas. Contrib Mineral Petrol 70:9–21
Dal Negro A, Carbonin S, Molin GH, Cundari A, Piccirillo EM (1982) Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional, and alkaline basaltic rocks. In: Saxena SK (ed) Advances in Physical Geochemistry Vol. 2. Springer, Berlin Heidelberg New York, pp 117–150
Dal Negro A, Carbonin S, Salviulo G. Piccirillo EM, Cundari A (1985) Crystal chemistry and site configuration of the clinopyroxene from leucite-bearing rocks and related genetic significance. The Sabatini lavas, Roman Region, Italy, J Petrol (in press)
Finger LW (1972) The uncertainty in the calculated ferric iron content of a microprobe analysis. Carnegie Inst Washington Yearb 71:600–603
James F, Roos M (1975) MINUIT, a system for function minimisation and analysis of the parameter errors and correlations. Computer Phys Commun 10:343–367
Papike JJ, Cameron K, Baldwin K (1974) Amphiboles and pyroxenes: characterisation of other than quadrilateral components and estimates of ferric iron from microprobe data. Geol Soc Am 6:1053–1054
Robinson K, Gibbs GV, Ribbe PH (1971) Quadratic elongation; a quantitative measure of distortion in coordination polyhedra. Science 172:567–570
Saxena SK, Dal Negro A (183) Petrogenetic application of Mg-Fe2+ order-disorder in orthopyroxene to the cooling history of rocks. Bull Mineral 106:443–449
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Dal Negro, A., Cundari, A., Piccirillo, E.M. et al. Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks. Contrib Mineral Petrol 92, 35–43 (1986). https://doi.org/10.1007/BF00373961
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DOI: https://doi.org/10.1007/BF00373961