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Introduction and previous work
Since the first determinations of the crystal structures of variscite (orthorhombic AlPO4⋅2H2O, Pbca) and topologically related metavariscite (monoclinic dimorph, P21/n) by Kniep et al. (1977) and Kniep and Mootz (1973) (redetermination, correcting a previous study of Fayos and Salvador-Salvador, 1971), respectively, a large number of both naturally occurring and synthetic members of these two groups have been reported. These members comprise predominantly phosphates and arsenates, but also vanadates (only one representative) and selenates (three representatives). An up-to-date compilation of the known minerals and synthetic compounds and their crystal data is provided in Tables 1 and 2 (variscite- and metavariscite-types, respectively), along with footnotes pointing out some errors and inconsistencies in the published data. This compilation reveals an interesting observation: while there are six variscite-type arsenate members, the metavariscite-type members include only one arsenate member (ScAsO4⋅2H2O). The topology of the framework-based variscite and metavariscite structure types was elucidated recently by Ilyushin and Blatov (2017), who also discussed the functional role of the hydrogen bonds within the frameworks (see also the discussion of topological features below).
Table 1. Overview on variscite-type minerals and synthetic compounds (five phosphates, five arsenates, three selenates).
| Mineral/Compound | Space group | a (Å) | b (Å) | c (Å) | V (Å3) | Reference |
| Variscite, AlPO4⋅2H2O | Pbca | 9.822(3) | 8.561(3) | 9.630(3) | 809.7 | Kniep et al. (1977) |
| Pbca | 9.821 | 8.558 | 9.622 | 808.81 | ICDD-PDF 33-33 | |
| Pcab | 9.83 | 8.56 | 9.64 | 811.2 | Salvador Salvador and Fayos (1972), ICDD-PDF 25-19 | |
| Strengite, FePO4⋅2H2O | Pbca | 8.720(1) | 9.879(1) | 10.119(2) | 871.7(3) | Kolitsch (unpublished SXRD refinement) |
| Pbca | 8.722(3) | 9.878(2) | 10.1187(14) | 876.8 | Taxer and Bartl (2004) | |
| (syn.) | Pbca | 9.8674(11) | 10.0973(11) | 8.7046(10) | 867.27(17) | Song et al. (2002b) |
| Pcab | 10.12 | 9.886 | 8.723 | 872.91 | ICDD-PDF 33-667 | |
| Pcab | 10.05 | 9.92 | 8.74 | 871.3 | Arlidge et al. (1963); ICDD-PDF 15-513 | |
| (Al-rich) | Pcab | 9.99 | 9.78 | 8.66 | 846.1 | Arlidge et al. (1963); ICDD-PDF 15-391 |
| Pcab | 8.65 | 10.05 | 9.80 | 851.9 | Kleber and Weiner (1958) | |
| n.d. | 8.65 | 10.06 | 9.84 | 856.3 | Kokkoros (1939) | |
| GaPO4⋅2H2O | Pbca | 9.9260(1) | 8.6189(1) | 9.7622(1) | 835.17 | Loiseau et al. (1998) |
| InPO4⋅2H2O | Pbca | 10.36 | 8.84 | 10.19 | 933.2 | Mooney-Slater (1961) |
| Pbca | 8.842(2) | 10.1870(10) | 10.327(2) | 930.19 | Xu et al. (1995) | |
| TlPO4⋅2H2 |