MANAKSITE (NaKMnSi4O10) AND FENAKSITE (NaKFeSi4O10): TWO SILICATES FROM THE HYPERAGPAITIC ROCKS OF LOVOZERO AND KHIBINA (KOLA PENINSULA, RUSSIA) WITH A PECULIAR [Si4O10]-4 CHANNEL-LIKE ANION

M. Prencipe1, E.A. Gorchakova2, G. Ferraris1 and A.P. Khomyakov3

1Dpt. Sci. Mineral. Petrol., Univ. Torino, Italy - ferraris@dsmp.unito.it
2Dpt. Geology, Univ. Moscow, Russia
3IMGRE-RAS, Moscow, Russia

 

Keywords: manaksite, fenaksite, silicates, Kola Peninsula, crystal structure

Manaksite (NaKMnSi4O10) and fenaksite (NaKMnSi4O10) are two silicates occurring in the hyperagpaitic rocks respectively of the Lovozero and Khibina massifs which constitute a complex showing a unique geochemistry and bear more than 500 different minerals, including over 100 new species (Khomyakov, 1995). Several of these minerals are silicates characterized by a large alkalis/(other cations) ratio and based on peculiar silicate anions. Manaksite was first described by Khomyakov et al. (1992) and, on the basis of chemical composition and cell parameters, was given as isotypic with fenaksite. An approximate crystal structure of fenaksite was earlier reported by Golovachev et al. (1970) on the basis of the following non reduced cell: a = 6.98, b = 8.24, c = 9.98 A, a = 114. 20, b = 80.23, g = 115.60°.

From samples collected in the original localities, crystals of manaksite and fenaksite suitable for single-crystal X-ray diffraction were selected and four-circles X-ray diffractometric data were collected both in Moscow (P21 Syntex) and in Torino (P4 Siemens and Italstructures) by using graphite-monochromatized MoKa radiation. Both structures were independently solved ab initio by direct methods and refined to R = 0.036 for 1850 (manaksite) and R = 0.026 for 1980 (fenaksite) independent reflections (SHELXL package). The two silcates turn out to be strictly isotypic, being the Fe/Mn substitution the only remarkable difference in the two crystal structures. They crystallize in the space group P1- with the following values for their reduced cells (obtained from the same set of 45 reflections): a = 6.987(1), b = 8.205(2), c = 10.032 A, a = 104.97, b = 101.01, g = 114.57° for manaksite; a = 6.983, b = 8.161, c = 9.957 A, a = 104.96, b = 101.01, g = 114.30° for fenaksite.

The main features of the two crystal structures are:

  1. the presence of a rare case of M = Fe/Mn fivefold coordinated, according to five independent M-O bonds ranging from 2.041 to 2.302 A; average value 2.155 and 2.141 A in manaksite and fenaksite, respectively;
  2. the alkaline K cation is eightfold coordinated with K-O bond lengths in the range 2.737-3.154 A; two next-neighbour oxygens occur in the range 3.4-3.6 A;
  3. the alkaline Na cation is seven-coordinated with Na-O in the range 2.382-3.070 A;
  4. the silicatic anion [Si4O10]-4 is very peculiar and consists of channels formed by four-members rings of Si-tetrahedra with a remarkable narrow range of average Si-O distances, 1.616-1.621 A; this type of configuration for the [Si4O10]-4 anion is known only for these two silicates;
  5. in spite of the presence of very distorted coordination polyhedra for the alkaline cations and of the unusual fivefold coordinated M cations (normally octahedral), charge and bond strength distributions are remarkably satisfactory.

 

  1. Golovachev V.P., Drozdov Yu.N., Kuz'min E.A. & Belov N.V. (1970). DAN USSR, 194, 818-820
  2. Khomyakov, A.P., Kurova T.A. & Nechelystov G.N. (1992). ZVMO., 121, 112-115.
  3. Khomyakov, A.P. (1995): Mineralogy of hyperagpaitic alkaline rocks. Clarendon Press, Oxford.