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The roedderite-chayesite series from Spanish lamproites: crystal-chemical characterization

Published online by Cambridge University Press:  05 July 2018

E. Alietii ,
M. F. Brigatti ,
S. Capedri  and
L. Poppi
E. Alietii
Affiliation:
Dipartimento di Scienze della Terra, Università di Modena, Via S. Eufemia, 19, I-41100 Modena, Italy
M. F. Brigatti
Affiliation:
Dipartimento di Scienze della Terra, Università di Modena, Via S. Eufemia, 19, I-41100 Modena, Italy
S. Capedri
Affiliation:
Dipartimento di Scienze della Terra, Università di Modena, Via S. Eufemia, 19, I-41100 Modena, Italy
L. Poppi
Affiliation:
Dipartimento di Scienze della Terra, Università di Modena, Via S. Eufemia, 19, I-41100 Modena, Italy
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Abstract

Members of the roedderite-chayesite series in lamproites from Cancarix (SE Spain), crystallized from late magmatic residua under low PH2O, high temperature (∼ 1100°C), oxidizing conditions. They exhibit the following main chemical variations: 0.14 ⩽ Na ⩽ 0.62 atoms per formula unit (apfu); 0.80 ⩽ K⩽ 1.00 apfu; 2.97 ⩽ Mg ⩽ 4.33 apfu; 0.00 ⩽ Fe2+ ⩽ 1.19 apfu; 0.42 ⩽ Fe3+ ⩽ 0.87 apfu; they are hexagonal (10.120 ⩽ a ⩽ 10.135 Å, 14.305 ⩽ c ⩽ 14.326 Å), P6/mcc. The characteristic chemical substitution is: Fe3+ + □ ⇌ Fe2+ + (K+Na)+. Six crystal structures have been refined to 0.020 ⩽ Robs ⩽ 0.026. They have the osumilite/milarite-type structure, with Si entering the double tetrahedral T1 six-membered rings, and Mg and Fe entering both the ring-linking T2 tetrahedra, and the A octahedra. The 12-coordinated C site, located between two double rings of T1 tetrahedra, is occupied mainly by K and subordinately by Na. Furthermore, Na occupies the partially empty nine-fold coordinated B site which occurs both in the ideal (z = 0) and in a split-atom position along the c direction.

Keywords

roedderitechayesitemilarite grouplamproiteCancarixSpain
Type
Mineralogy
Information
Mineralogical Magazine , Volume 58 , Issue 393 , December 1994 , pp. 655 - 662
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1994

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