On the Isomorphism of Sodium at the M (2) Site in Eudialyte-Group Minerals: The Crystal Structure of Mn-Deficient Manganoeudialyte and the Problem of the Existence of the M (2) Na-Dominant Analogue of Eudialyte
Sodium plays an important role in the crystal structures of eudialyte-group minerals given that it can occupy different crystallographic sites. Predominantly, it distributes between the N (1–5) sites situated in the large cavities of the heteropolyhedral framework. Rarely, Na occupies split sites of...
Published in: | Minerals |
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Main Authors: | , , , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
MDPI AG
2022
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Subjects: | |
Online Access: | https://doi.org/10.3390/min12080949 https://doaj.org/article/cd03299360a348e7a6d98d355ecd9e53 |
Summary: | Sodium plays an important role in the crystal structures of eudialyte-group minerals given that it can occupy different crystallographic sites. Predominantly, it distributes between the N (1–5) sites situated in the large cavities of the heteropolyhedral framework. Rarely, Na occupies split sites of the M (2) microregion where it can predominate over other elements (predominantly Mn, Fe 2+ , and Fe 3+ ). The crystal structure of the Mn-deficient manganoeudialyte from the Lovozero alkaline complex (Kola Peninsula, Russia) has been refined. The trigonal unit–cell parameters are: a = 14.1848(2) Å, c = 30.4726(3) Å, V = 5309.90(11) Å 3 . The sample is a rare example of a high-sodium and high-calcium representative of the eudialyte group with Fe + Mn < 2 apfu . The idealized formula is Na 14 Ca 6 [(Mn,Fe) 2 Na]Zr 3 Si 2 [Si 24 O 72 ]O(OH)·2H 2 O with bivalent components, Mn 2+ and Fe 2+ , dominating at the M (2) site. The regularities of isomorphism involving M (2) Na in EGMs and the problem of the existence of the M (2) Na-dominant analogue of eudialyte are discussed. The new data obtained in this work confirm the previous conclusion that the complete isomorphism between Ca-deficient and Ca-rich members of the eudialyte group cannot be realized in frames of a single-space group ( R 3 m , R- 3 m or R 3). Thus, the existence of the M (2) Na analogue of eudialyte remains questionable. |
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