Feldspars are among the most common phases in the Earth’s crust and are characterized by wide variations in chemical composition and crystal structure motifs. In this paper, we synthesized SrZn2P2O8 and studied it by means of scanning electron microscopy, electron microprobe analysis and powder X-ray diffraction at ambient and high-temperature (up to 900°C) conditions. We have structurally characterized for the first time layered triclinic β-modification of SrZn2P2O8 (space group P$\bar 1$, a = 5.0109(8), b = 8.6202(13), c = 9.7527(15) Å, α = 118.088(3), β = 74.622(6), γ = 87.525(6)° and V = 351.29(10) Å3), which is structurally related to hexacelsian. The high-temperature study demonstrated that β-SrZn2P2O8 transformed irreversibly into the framework monoclinic paracelsian-like α-SrZn2P2O8 (space group P21/c, a = 8.3163(2), b = 9.5023(2), c = 9.0248(2) Å, β = 92.285(1)°, V = 712.60(1) Å3) above 600°C, which, in turn, is stable up to 900°C, when it melts. Though the crystal structures of α- and β-modification of SrZn2P2O8 are different (framework and layered, respectively), their thermal expansion is similar: αV ≈ 26 ± 1 × 10–6 °C–1, αmax/αmin ≈ 5. The possible occurrence of the corresponding minerals in nature is discussed.

Feldspars are the most common minerals in the Earth’s crust, and commonly contain trace concentrations of gallium and germanium. However, their gallium/germanium synthetic analogues are widely used, both in industrial applications and geological reconstructions. Here KMeGe3O8 (Me = Al, Ga) compounds were synthesized using solid-state reactions and hydrothermal synthesis and studied by microprobe analysis and powder X-ray diffraction (under both ambient and high-temperature conditions). In both (Al and Ga) cases, paracelsian-like phases were the first to form, even though feldspar-like phases are predicted to be thermodynamically stable. Paracelsian-like phases are transformed irreversibly upon heating (930 ± 30 and 960 ± 30°C for KGaGe3O8 and KAlGe3O8, respectively) into feldspar-like modifications. According to the obtained data, the feldspar-like modifications exhibit a higher degree of anisotropy, whereas the volume thermal expansion coefficients are higher for paracelsian-like modifications. The influence of chemical composition and topology on the crystallization process, thermal transformation and thermal expansion of feldspar- and paracelsian-like compounds is discussed.

The crystal structure of KBSi3O8 (orthorhombic, Pnam, with a = 8.683(1), b = 9.253(1), c = 8.272(1) Å,, V = 664.4(1) Å3, Z = 4) has been determined by the direct method applied to 3- dimensional rcflection data. The structure of a microcrystal with the dimensions 20 × 29 × 37 μm was refined to an unweightcd residual of R = 0.031 using 386 non-zero structure amplitudes. KBSi3O8 adopts a structure essentially different from recdmergneritc NaBSi3O8, with the low albite (NaAlSi3O8) structure, and isotypic with danburite CaB2Si2Os which has the same topology as paracelsian BaAl2Si2O8. The chenfical relationship between this sample and danburitc gives insight into a new coupled substitution; K+ + Si4+ = Ca2+ + B3+ in the extraframework and tetrahedral sites. The present occupancy refinement revealed partial disordering of B and Si atoms which jointly reside in two kinds of general equivalent points, T(1) and T(2) sites. Thus the expanded crystal-chemical formula can be written in the form K(B0.44Si0.56)2(B0.06Si0.94)2O8

The systematic trend among crystalline compounds with the M+T3+T4+ 3O8 formula suggests that they exist in one of four structural types; the feldspar structures with T3+/T4+ ordered and/or disordered forms, and the paracelsian and the hollandite structures.