Distribution of alkali cations in some beryllium minerals from granitic pegmatites

Yakubovich O.V.*, Pekov I.V.*^,**

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

yakubol@geol.msu.ru

               The minerals that contain Cs together with light elements, Li, Be and B, namely that of a solid-solution rhodizite-londonite series with idealized end-members formulae KAl₄Be₄(B₁₁Be)O₂₈ - CsAl₄Be₄(B₁₁Be)O₂₈, and silicates of the beryl group like a Li,Cs-variety of beryl, vorobyevite, (Cs,Na)_x(H₂O)_n(Be_3-xLi_x)Al₂Si₆O₁₈, and Be,Li-ordered individual species pezzottaite, (Cs,Na)(H₂O)_n(Be₂Li)Al₂Si₆O₁₈, are suggested for considering as indicators of a specific type of rare-element granitic pegmatites, where these minerals are geochemically important concentrators of caesium.

               The crystal structures of microporous minerals vorobyevite, pezzottaite and londonite from granitic pegmatites have been studied using single-crystal X-ray diffraction at a low temperature. It is confirmed that, at a high lithium content in minerals of the beryl group, this alkali metal is selectively incorporated into Be tetrahedra. The positive charge deficit due to the substitution of Be²⁺ for Li⁺ cations is compensated by capturing large alkali cations into the channel of the aluminosilicate quasi-framework. In the situation of Li content close to unity per formula, the Li and Be atoms become ordered with a formation of the rhombohedral crystal structure of pezzottaite. The example of the vorobyevite-pezzottaite pair demonstrates that, in a beryl-type structure, Be and Li atoms can be arranged in both a disordered and a completely ordered manner. Vorobyevite can be considered as a beryl variety (it retains the beryl structure type, e.g. the space group and the unit cell metric), which is characteristic for gem- and rare-metal-bearing type of granitic pegmatites: it is enriched in both lithium replacing beryllium and caesium located in channels. The main substitution scheme describing the transition from “common” beryl to vorobyevite is identical to that for the beryl-pezzottaite transformation: Li⁺ + Cs⁺ ® Be²⁺ + (,H₂O).

The ordering of Li and Be atoms in the rhombohedral structure of pezzottaite is accompanied by a change in the sizes of the corresponding tetrahedra. The aforementioned character of their distortion and distribution in the crystal structure (alternation according to the scheme …–Li–Be–Be–Li–Be–Be–…) along the c axis and the a' and b' pseudo-axes correlates with the formation of two types of silicon–oxygen six-membered rings formed by the Si1 (the first type) or Si2/Si3 (the second type) tetrahedra. The ratio between the numbers of rings of the first and second types in the pezzottaite structure corresponds to the ratio between the numbers of Li and Be tetrahedra and amounts to 1 : 2. Compared to vorobyevite, pezzottaite is characterized by higher contents of lithium cations, as well as large alkali cations in the channels due to the requirement of the charge balance. In the pezzottaite studied, these “zeolitic” cations are represented by Cs and Na in a ratio of approximately 3 : 1. The presence of the Na⁺ cations requires the incorporation of water molecules “completing” the coordination polyhedra of sodium atoms in the structure. The H₂O molecules are located in the immediate vicinity of the positions partially occupied by the Cs atoms, and the number of these atoms uniquely determines the maximum possible water content. The independent refinement of the numbers of oxygen atoms of water molecules and sodium atoms showed that they coincide with each other (within the limits of experimental error): the total number of Na atoms turns out to be almost identical to the total number of water molecules.

The crystal structure of londonite is based on an incomplete array of cubic close-packed oxygen atoms. Some of the octahedral voids inside a defective close-packing of oxygen atoms (28 atoms instead of 32 possible in the unit cell) are occupied by Al³⁺ cations. The B and Be atoms are ordered at two different positions with tetrahedral coordination; the boron position contains a small amount of vacancy defects, while the beryllium position is “diluted” by boron. Four edge-sharing AlO₆ octahedra form clusters that are linked by BO₄ and BeO₄ tetrahedra in a microporous quasi-framework with cages located at the cell centers. The large alkali cations Cs⁺ and K⁺ occupy distinct positions in these cages, and both are coordinated by 12 oxygen atoms. The structural formula of londonite derived from this model is Cs_0.55K_0.33{Al₄[Be_3.64(B_10.80Be_1.20)O₂₇(OH,F)]}. One hydroxyl group together with an insignificant amount of F^– found by the electron microprobe is necessary to achieve charge balance. The occurrence of a small amount of hydroxyl is confirmed by IR spectroscopy data for rhodizite. We found that Cs and K are ordered in londonite from the Urals, in contrast to their random distribution reported for Cs-rich rhodizite from Madagascar. In the structure of londonite from the Urals, Cs⁺ cations occupy the 1b special positions, where all alkali cations were placed in earlier studies. The K⁺ cations are moved toward the cage walls compared to the position of Cs⁺ cation at the cage center. The very short interatomic Cs–K distances, 0.51(3) Å, prevent the simultaneous occupancy of these positions in the same cage. Our refinement shows that the alkali atoms are distributed statistically in the following ratios: Cs_0.55K_0.33.

It is important to note that the proportion of vacancies at the Be position is correlated with the amount of K. In addition to the complementary occupancy of K and Cs, Be and K are also separated by an unusually short distance, 2.76(3) Å, preventing their simultaneous occupancy. The capacity of these minerals for K is strongly coupled with the Be content in the 4e position, and is variable. The crystal structure of londonite and evidently that of rhodizite are stabilized by vacancy defects at these Be positions. The bond-valence calculation shows that bond strengths for oxygen atoms that participate in the coordination of beryllium (O1 and O3) and the alkali cations (O3) are nearly equal to 2. Therefore, significant changes in the Be content would infringe upon the structure stability. Thus, the deficiency of large alkali cations from 1 apfu in rhodizite and londonite, as found earlier and confirmed in this research is not accidental.

Interesting structural relationships have been revealed by comparing the structure type of rhodizite–londonite with that of pharmacosiderite. The rhodizite–londonite structure may be considered formally to be built from clusters of four octahedra sharing edges (elements of the pharmacosiderite structure), and a sodalite-type framework of tetrahedra. The association of these two essential structural fragments, [M₄O₄] and [T₁₂O₂₄] (M: octahedron, T: tetrahedron), that have been found in the pharmacosiderite and sodalite structures, results in the formation of an original complex, [Al₄O₄B₁₂O₂₄]^8– in londonite. The Be²⁺ and alkali cations fill the interstices and compensate for its anionic charge. This interpretation treats Be, Cs and K as guest (but necessary) components in the composition of rhodizite–londonite series minerals. Thus, vacancy defects at the Be, Cs and K positions in rhodizite and londonite are consistent with its structural “genealogy”.

               This work is financially supported by RFBF grants. 10-05-01068-a, 08-07-00077-a, 09-05-12001-ofi_m and 10-05-91333-NNIO_a  and grant of President of Russian Federation no. NSh-3848.2010.5.