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Kurchatovite and clinokurchatovite, both of ideal composition CaMgB 2 O 5 , from the type localities (Solongo, Buryatia, Russia, and Sayak-IV, Kazakhstan, respectively) have been studied using electron microprobe and single-crystal X-ray diffraction methods. The empirical formulae of the samples are Ca for kurchatovite and clinokurchatovite, respectively. The crystal structures of the two minerals are similar and based upon two-dimensional blocks arranged parallel to the c axis in kurchatovite and parallel to the a axis in clinokurchatovite. The blocks are built up from diborate B 2 O 5 groups, and Ca 2+ and Mg 2+ cations in seven-and six-fold coordination, respectively. Detailed analysis of geometrical parameters of the adjacent blocks reveals that symmetrically different diborate groups have different degrees of conformation in terms of the δ angles between the planes of two BO 3 triangles sharing a common O atom, featuring two discrete sets of the δ values of ca. 55 • (B' blocks) and 34 • (B" blocks). The stacking of the blocks in clinokurchatovite can be presented as . . . (+B')(+B")(+B')(+B") . . . or [(+B')(+B")], whereas in kurchatovite it is more complex and corresponds to the sequence . . . (+B')(+B")(+B')(−B')(−B")(−B')(+B')(+B")(+B')(−B')(−B")(−B'). . . or [(+B')(+B")(+B')(−B')(−B")(−B')]. The B':B" ratios for clinokurchatovite and kurchatovite are 1:1 and 2:1, respectively. According to this description, the two minerals cannot be considered as polytypes and their mutual relationship corresponds to the term modular polymorphs. From the viewpoint of information-based measures of structural complexity, clinokurchatovite (I G = 4.170 bits/atom and I G,total = 300.235 bits/cell) is structurally simpler than kurchatovite (I G = 4.755 bits/atom and I G,total = 1027.056 bits/cell). The high structural complexity of kurchatovite can be inferred from the modular character of its structure. The analysis of structural combinatorics in terms of the modular approach allows to construct the whole family of theoretically possible "kurchatovite"-type structures that bear the same structural features common for kurchatovite and clinokurchatovite. However, the crystal structures of the latter minerals are the simplest and are the only ones that have been observed in nature. The absence of other possible structures is remarkable and can be explained by either the maximum-entropy of the least-action fundamental principles.

Single crystals of the novel boroarsenate (K,Na) 2 [As 2 B 12 O 24 ][B 3 O 3 (OH) 3 ] (I) were obtained using the borax flux method. The crystal structure of I was found to be triclinic, P-1, a = 8.414(5), b = 10.173(6), c = 15.90(1) Å, α = 79.56(1), β = 78.68(1), γ = 70.91(1), V = 1251(1) Å 3 , Z = 2. The crystal structure of I is based upon the novel [AsB 6 O 12 ] − microporous boroarsenate framework formed by B and As coordination polyhedra. This framework can be subdivided into borate units that are interlinked by AsO 4 tetrahedra. In the case of I, the borate substructure is a chain consisting of triborate rings, 2∆, formed by two (BO 3 ) triangles and one (BO 4 ) tetrahedron connected through shared common oxygen atoms. The chains are extended along [011] and are interlinked by (AsO 4 ) tetrahedra in the [011] direction. As a result, the framework has large channels parallel to [100], having an effective diameter of 4.2 × 5.6 Å 2 . The channels contain occluded electroneutral ring triborate complexes, [B 3 O 3 (OH) 3 ] 0 , formed by three (BO 2 (OH − )) − triangles sharing common O atoms, as well as K + and Na + cations. The triborate [B 3 O 3 (OH) 3 ] 0 units correspond to similar clusters found in the crystal structure of the α-form of metaboric acid, HBO 2 . According to information-based complexity calculations, the crystal structure of I should be described as complex, with I G = 5.781 bits/atom and I G,total = 625.950 bits/cell. Teruggite, Ca 4 Mg[B 6 As(OH) 6 O 11 ] 2 (H 2 O) 14 , the only known boroarsenate of natural origin, has almost twice as much information per unit cell, with I G,total = 1201.992 bits/cell. The observed difference in structural complexity between I and teruggite is the consequence of their chemistry (hydration state) and different formation conditions.