Chemistry of cancrinite-group minerals OF different FORMATIONS

Olysych L.V.*, Pekov I.V.*^,**, Chukanov N.V.***

*Moscow State University, Moscow, Russia;

 **Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow,Russia;

***Institute of Problems of Chemical Physics, Chernogolovka, Russia

The minerals of the cancrinite group (CGM) are hexagonal and trigonal tectoaluminosilicates with extra-framework anions. Their general formula is: A_7-8[Al_6-ySi_6+yO₂₄]X_1-4žnH₂O, with extra-framework cations A = Na, K, Ca (with subordinate Sr and Ba) and extra-framework anions X = (CO₃)^2-, (SO₄)^2-, Cl^-, (OH)^-, S^2-, (C₂O₄)^2- (with subordinate F^- and (PO₄)^3-); y < 1.2, n = 0–5. Frameworks of CGM consist of coplanar interrupted layers of six-membered rings of SiO₄ and AlO₄ tetrahedra. The stacking of these layers determines a framework with wide channels and small undecahedral cages which host extra-framework cations and anions and H₂O molecules.

There are 21 minerals of the cancrinite group up to date. Six of them, namely cancrinite Na₆Са[Si₆Al₆O₂₄](CO₃)ž2H₂O, cancrisilite Na₇[Si₇Al₅O₂₄](CO₃)ž3H₂O, hydroxycancrinite Na₈[Si₆Al₆O₂₄](OH)₂ž2H₂O, vishnevite  Na₆K₂[Si₆Al₆O₂₄](SO₄)ž2H₂O, kyanoxalite Na₇(Al_6-5Si_6-7O₂₄)(C₂O₄)_0.5-1ž5H₂O and davyne Na₄K₂Са₂[Si₆Al₆O₂₄]Cl(SO₄,CO₃)₂ž2H₂O can be considered as a cancrinite ss subgroup minerals: all of them have a small unit cell (a = 12.5–12.7, c = 5.1–5.4 å).

We have discussed chemical data on 400 samples of all CGM from worldwide localities: 270 our samples  and 130 earlier published analyses. All CGM can be subdivided into three main genetic types: 1) intrusive alkaline complexes: magmatic rocks, pegmatites, metasomatic and hydrothermal bodies, 2) alkaline volcanic complexes and 3) metamorphic and metasomatic formations not related to the alkaline magmatism. CGM of the first genetic type (formation) are the most widespread. They are represented by cancrinite ss subgroup minerals with the only exception  Figure 1. Correlation between the Al/Si ratio and content of bivalent extra-framework cations (mainly Ca) in CGM of three genetic types: 1) intrusive alkaline complexes, 2) alkaline volcanic complexes and 3)  formations not related to the alkaline magmatism.

of davyne uncommon for this type of rocks but typical for volcanic complexes and formations not related with alkaline magmatism.  Some CGM (cancrinite and vishnevite) are rock-forming minerals in intrusive alkaline complexes. CGM of the second and third genetic types are mainly rare minerals with large unit cells. Davyne is the only representative of the cancrinite ss subgroup in these two formation types.

The chemical diversity of CGM from intrusive alkaline complexes can be presented as:

1. A cancrinite–cancrisilite solid-solution series with general scheme of substitutions: Al³⁺ + Ca²⁺ + 0.5(CO₃)^2- ↔ Si⁴⁺ + ? + H₂O. The minerals of this series reveal distinct positive correlation between Al/Si and Na/Ca ratios (Fig.1) while the extra-framework anion substitutions are subordinate.

2. A cancrinite–vishnevite isomorphous series with a break between members with (SO₄)_0.36­­ and (SO₄)_0.56. As the SO­_4­-group incorporates in cancrinite structure, the correlation between Al/Si and Na/Ca raties is violated and another scheme is realized: 2(Na,K)⁺ + (SO₄)^2- ↔ Ca²⁺ + (CO₃)^2- (Figs.1-2). It seems crystallochemically

Figure 2. Correlation between (SO₄)^2- and Cl^- in CGM of three genetic types: 1) intrusive alkaline complexes, 2) alkaline volcanic complexes and 3)  formations not related to the alkaline magmatism; C-CGM are species with prevailing C-bearing (carbonate or oxalate) extra-framework anions.

important that in some cases cancrinite is closely associated with vishnevite (Vishnevye Gory and Ilmeny, both Urals, Russia) and they are significantly different chemically: cancrinite is Ca-enriched (4.3-6.8 wt.% СаО that corresponds to 0.8-1.2 Ca apfu) and K-poor (0-0.4 wt.% К₂О: 0-0.1 2 K apfu) while vishnevite, quite the contrary, is represented by K-rich variety (8.1-8.7 wt.% К₂О: 1.8-2.1 К apfu) and Ca-poor (0.5-0.9 wt.% СаО: 0.1-0.2 apfu). In these assemblages, cancrinite can contain SO₄ (up to 0.3 per formula: 2.4 wt.% SO₃), as well as IR-spectra of vishnevite shows considerable amount of CO₃ groups. 

3. A cancrinite–kyanoxalite solid solution series with the general scheme of substitutions: Al³⁺ + Ca²⁺ + (CO₃)^2- ↔ Si⁴⁺ + Na⁺ + (С₂O₄)^2-. The Al/Si ratio in minerals of this series varies from 0.73 to 0.85.

4. A four-component solid solutions system (with breaks) of Ca-poor ([Ca+Sr]<0.2 apfu) minerals. The components of this system are cancrisilite, hydroxycancrinite, vishnevite and kyanoxalite (Fig. 1). The most important characteristics here are Al/Si ratio (varies from Al_6.3Si_5.7 to Al_4.4Si_7.6) and the type of extra-framework anion. This system was found for peralkaline sodic complexes, first Lovozero (Kola Peninsula, Russia).

The majority of CGM from intrusive alkaline complexes are represented by Cl-poor to Cl-free CO₃- and SO₄-dominant species (Fig.2).

Chemical composition of CGM from alkaline volcanic complexes is quite different (Fig. 1). Unlike CGM from intrusive complexes (genetic type 1), these minerals are Ca-rich (1.7-3.6 apfu:10.2-16.7 wt.% СаО), with the Al/Si ratio variations from 0.9 to 1.0 (Fig. 1). Their prevaling extra-framework anions are SO­₄^2- and Cl^- showing distinct negative correlation (Fig. 2). CO₃-dominant CGM are almost absent in this formation.

Chemical composition of CGM from the formations not related to the alkaline magmatism (first from lazurite deposits) is similar to that from alkaline volcanic rocks. These minerals are also Ca-rich and typically have SO­₄^2- and Cl^- as species-defining anions but their composition field is not so wide (2.4 ≤ ∑A²⁺ ≤ 2.8 apfu: Figs. 1 and 2).

This study was supported by grants of RFBR Nos. 07-05-00130-a, 08-05-00077-а and 09-05-91330-NNIO_а, grant of President of Russain Federation No. 863.2008.5 and grant of Russian Science Support Foundation (I.V.P.).