High temperature polymorphic transition and crystal structures of feldspar-related SrZn ₂ P ₂ O ₈

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 SrZn 2 P 2 O 8 and studied it by means of scanning electron microscopy, electron microprobe analysis, powder X-ray diffraction at ambient and high-temperature (up to 900 C) conditions.We have structurally characterized for the first time layered triclinic -modification of SrZn 2 P 2 O 8 (space group P , a = 5.0109(8), b = 8.6202(13), c = 9.7527(15) , = 118.088(3), = 74.622(6), = 87.525(6),V = 351.29(10) 3 ), which is structurally related to hexacelsian.The high-temperature study demonstrated that -SrZn 2 P 2 O 8 irreversibly transformed into framework monoclinic paracelsian-like -SrZn 2 P 2 O 8 (space group P2 1 /c, a = 8.3163(2), b = 9.5023(2), c = 9.0248(2) , = 92.285(1),V = 712.60(1) 3 ) above 600 C, which, in turn, is stable up to 900 C, when it melts.Though the crystal structures of -and -modification of SrZn 2 P 2 O 8 are different (framework and layered, respectively), their thermal expansion is similar: V ~ 26 1 10 -6 C-1 , max / min ~5.The possible occurrence of the corresponding minerals in nature is discussed.