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

• Published in: Minerals
• Main Authors: Sergey M. Aksenov, Nikita V. Chukanov, Igor V. Pekov, Yulia V. Nelyubina, Dmitry A. Varlamov, Lia N. Kogarko
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2022
• Subjects: eudialyte-group minerals; blocky isomorphism; solid solution; alkaline rocks; manganeudialyte; Mineralogy; QE351-399.2; Kola Peninsula; Lovozero; kola peninsula
• Online Access: https://doi.org/10.3390/min12080949; https://doaj.org/article/cd03299360a348e7a6d98d355ecd9e53

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.