High-pressure and phase transitions in dalyite, a Zr-silicate Available to Purchase

Dalyite, ideally K₂ZrSi₆O₁₅, Sp. Gr P-1, is a rare potassium zirconium silicate found in agpaitic alkaline rocks, i.e., rocks with (Na + K)/Al ≥ 1.2 in which complex zirconosilicate minerals can form. Dalyite structural evolution and compressibility were studied by in-situ single-crystal synchrotron X-ray diffraction up to 20.52(5) GPa. In the P-range investigated we observed: (i) a second order (distorsive) phase transition, from dalyite to dalyite-II (Sp. Gr P-1), between 9.90(5) and 10.68(5) GPa and (ii) a first-order phase transition, from dalyite-II to dalyite-III (Sp. Gr P-1), between 11.06(5) and 12.03(5) GPa. Fitting the PV data with a second-order Birch–Murnaghan EoS (BM2) we obtained a value of K_V0 = 51.4(4) GPa for dalyite (V₀ = 332.5(2)ų) and K_V0 = 39(1) GPa for dalyite-III (V₀ = 670(4)ų); due to the narrow stability field of the dalyite-II polymorph (∼1.3 GPa) its elastic parameters could not be determined. The structure accommodates the deformation induced by pressure by increasing anisotropy, from dalyite having a strain ellipsoid axial value of ɛ₁:ɛ₂:ɛ₃∼1.69:1.28:1 (calculated between 0.22(5) and 9.90(5) GPa) to dalyite-III with ɛ₁:ɛ₂:ɛ₃∼6.4:2.0:1 (calculated between 12.03(5) and 20.52(5) GPa). Both phase transitions have been found perfectly reversible in character and the high pressure polymorph structures have been solved. From dalyite to dalyite-II and dalyite-III the unit-cell doubled and the coordination of K polyhedra increases from eight-to nine-fold. Interestingly, before the first phase transition the SiO₄ tetrahedra have a significant role in accommodating the bulk compression and show different bulk compressibility (=47(4) GPa, =74(14) GPa and =108(20) GPa), but after the transition the compressibility is accommodated mainly by deforming the ZrO₆ octahedra, the β-wollastonite chains and the 4- and 6-mRs tetrahedra rings. The deformation of the SiO₄ tetrahedrons and Zr-polyhedrons constitutes a rather peculiar and unusual behaviour, rarely observed before in high-pressure studies.