The crystal structures of coquimbite and paracoquimbite from the Hongshan Cu–Au deposit, NW China, were refined by single-crystal X-ray diffraction, including the positions of all non-H atoms and hydrogen atoms.

The unit-cell parameters of coquimbite are a = 10.9644(15), c = 17.090(3) Å, and V = 1769.6(5) Å3, Z = 4 with space group P3¯1c. The crystal structure was refined based on 1352 unique reflections to R1(F) = 0.0301, yielding the formula Fe1.64Al0.36(SO4)3∙9H2O. Among the three cation positions of coquimbite, the isolated octahedral one is dominantly occupied by Al, i.e. Al0.660(3)Fe0.340(3). Consequently, the average Al–O interatomic distance is 1.9155 Å, smaller than the Fe–O distances, 2.002 Å for Fe(1), and 1.989 Å for Fe(2). Based on the structure data available for coquimbite in the literature, the ideal formula should be expressed as AlFe3(SO4)6·18H2O (Z = 2) or Fe2−xAlx(SO4)3·9H2O, x ∼ 0.5 rather than Fe3+2(SO4)3∙9H2O. The simplified formula for coquimbite from Hongshan is (Al0.66Fe0.34)Fe3(SO4)6·18H2O.

The unit-cell parameters of paracoquimbite are a = 10.9631(16), c = 51.473(10) Å, and V = 5357.7(15) Å3, Z = 12 with space group R3¯⁠. The crystal structure was refined using 2733 unique reflections to R1(F) = 0.0550, which led to the formula Fe1.91Al0.09(SO4)3∙9H2O. There are five non-equivalent octahedrally coordinated metal sites for Fe in the structure of paracoquimbite. The range of Fe[6]–O interatomic distances in paracoquimbite (1.966 Å–2.016 Å) can well be compared with those found in various sulfate minerals of ferric iron.

Both minerals are characterized by a complex system of hydrogen bonds involving three types of H2O molecules, respectively: those of [Fe3(SO4)6(H2O)6]3− clusters, of isolated [Fe(H2O)6]3+ or [Al(H2O)6]3+ octahedra and further interstitial (H2O) groups. The interstitial H2O molecules resemble a cyclohexane-like chair and are held in the structure solely by hydrogen bonding. In coquimbite H2OW(2) is disordered with two alternative orientations (H2B1 and H2B2), in paracoquimbite interstitial H2O consists of two H2O molecules, H2OW(3) and H2OW(4). The range of the O–H stretching frequencies between ∼2750 and ∼3650 cm−1 from infrared spectroscopy for paracoquimbite is in fair accordance with the calculated values between ∼2874 and ∼3600 cm−1 from both the d(O…O) lengths and the d(H…O) lengths.

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