Genetic crystal chemistry of cancrinite-group minerals

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

*Faculty of Geology, Moscow State University, Moscow, Russia; **Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia; ***Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia.

igorpekov@mail.ru

 

The cancrinite group includes 22 hexagonal and trigonal tectoaluminosilicates with additional (extra-framework) anions. The general formula of cancrinite-group minerals (CGM) is: A_7-8[Al_6-ySi_6+yO₂₄]X_1-4·nH₂O, with major extra-framework cations A = Na, K, Ca and species-defining extra-framework anions X = CO₃^2-, SO₄^2-, Cl^-, OH^-, S^2-, C₂O₄^2- 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 type of the framework with wide channels (or chains of wide, zeolite-like cavities) and narrow channels formed by cancrinite-type cages. Channels of both types host extra-framework cations A and anions X and, in majority of CGM, also H₂O molecules. The three possible positions of neighbouring layers in the xy plane are usually denoted by the letters A, B and C (Bonaccorsi, Merlino, 2005). CGM are widespread, sometimes even rock-forming components in alkaline rocks of different types.

CGM show the most number of variable crystal-chemical characteristics among all aluminosilicates. Main of these features are: Al/Si-ratio; general amounts and ratios of extra-framework constituents: cations, H₂O molecules and, especially, anions; type of framework that determines size and geometrical parameters of zeolite-like cavities; location of extra-framework cations, anions and H₂O molecules in both wide and narrow channels; character of coordination of additional anions by large cations in wide channels. It makes CGM ones of the most sensitive indicators of physical and chemical parameters of mineral-forming media in complexes of alkaline rocks, especially in pegmatites, hydrothermalites and metasomatites.

Among all CGM, six species, namely cancrinite, cancrisilite, hydroxycancrinite, vishnevite, kyanoxalite (Chukanov e.a., 2009) and depmeierite (Pekov e.a., in press) are characterized by the smallest unit cell with the parameters a » 12.6-12.8, c » 5.1-5.4 Å, a framework with the simplest stacking sequence AB and presence of Na (typically the chains ...-Na⁺-H₂O⁰-...) in narrow channels. These minerals, united by us to the cancrinite s.s. subgroup, occur only in alkaline intrusive massifs, except of very rare finds of cancrinite in effusive complexes. They were formed under relatively low temperatures (< 500ºC; in many cases < 250ºC) and high H₂O fluid pressure. In alkaline volcanic rocks of Eifel, W. Germany, we have found almost H₂O-free cancrinite that demonstrates important role of H₂O fluid pressure. Other 16 CGM were not found in intrusive alkaline complexes but they are typical for volcanic alkaline formation and formations not related directly to alkaline magmatism (first, metamorphic rocks with feldspathoids and lazurite-bearing metasomatites). They were formed under high temperature (typically > 500ºC) and, probably, low H₂O fluid pressure.

Members of the cancrinite s.s. subgroup are almost Cl-free unlike other CGM having strong affinity to Cl^- anion that forms the chains ...-Ca²⁺-Cl^--... in narrow channels. In sodalite-group minerals, vice versa, Cl^- forms stable extra-framework tetrahedral clusters not with Ca²⁺ but with Na⁺: [Na₄Cl]³⁺. Distribution of Cl between sodalite- and cancrinite-group minerals seems an indicator of the Na/Ca ratio.

Members of the cancrinite-cancrisilite solid-solution series, the carbonate-rich CGM, unlike majority of other tectoaluminosilicates, are sensitive indicators of agpaicity. We have found clear correlation between agpaicity and coupled heterovalent substitutions Na-Ca and Si-Al in these minerals. These substitutions significantly influence to the type of infrared spectrum of cancrinite-cancrisilite series members that makes their IR spectra usable indicator sign.

We have discovered strong affinity of CGM to oxalate extra-framework anion C₂O₄^2-. Its presence shows reduction conditions. Generally, CGM can contain two systems of addition anions indicating redox potential of mineral-forming media: there are the three S⁶⁺O₄^2- – S⁴⁺O₃^2- – S^2- and the pair CO₃^2- – C₂O₄^2-. It is unique for minerals, especially rock-forming ones.

In high-alkaline and Ca-poor systems, CGM are represented by almost Na-pure members or its K-enriched varieties or analogues. Their composition in part of extra-framework anions is sensible to activities of volatiles, except of halogens, in fluid phase. The brightest example is hyperagpaitic pegmatites and hydrothermalites of the Lovozero complex, Kola, Russia: all six members of the cancrinite s.s. subgroup occur there showing the diversity of species-defining extra-framework anions: CO₃^2-, SO₄^2-, OH^-, C₂O₄^2- and PO₄^3-. Cancrinite in Lovozero is represented by unusual almost Ca-free and C₂O₄-bearing variety forming the continuous solid-solution series with kyanoxalite.

This work was supported by grants nos. 08-07-00077-a and 10-05-91333-NNIO_a of RFBR and grants of President of Russian Federation nos. NSh-4034.2010.5 and NSh-3848.2010.5.

 

References:

Bonaccorsi E., Merlino S. Modular microporous minerals: cancrinite-davyne group and C-S-H phases // Reviews in Mineralogy and Geochemistry. 2005. Vol. 57. P. 241-290.

Chukanov N.V., Pekov I.V., Olysych L.V., Massa W., Yakubovich O.V., Zadov A.E., Rastsvetaeva R.K., Vigasina M.F. Kyanoxalite, a new cancrinite-group mineral with oxalate extra-framework anion from the Lovozero alkaline massif, Kola peninsula // Zapiski RMO. 2009. N. 6. P. 18-35 (in Russian).

Pekov I.V., Olysych L.V., Zubkova N.V., Chukanov N.V., Van K.V., Pushcharovsky D.Yu. Depmeierite, Na₈[Al₆Si₆O₂₄](PO₄,CO₃)_1-x·3H₂O (x < 0.5), a new cancrinite-group mineral from the Lovozero alkaline complex, Kola Peninsula, Russia // Zapiski RMO, in press.