F igure 10. XRD data showing kornelite transformation to paracoquimbite at 62% RH. Five lines indicate 0, 7, 23, 43, and 79 days preservation of kornelite powders at 62% RH from bottom to top. Percentages are the weight proportions of each phase estimated from Rietveld refinement. K = Kornelite

with the initials of corresponding mineral names and diffraction index: F = Ferricopiapite; R = Rhomboclase; K = Kornelite; P = Paracoquimbite. Note that the broad peak at 7° is produced by the scattering from Kapton capillary and the chamber wall. ( a ) M-Fe 2 (SO 4 ) 3 transformed to ferricopiapite

Fig. 5 Top view of the crystal structures of kornelite and lausenite, shown as polyhedral models (left) and topological schemes (right). In the topological schemes, only the cations are shown, Fe 3+ as grey circles and S 6+ as white circles. Dashed lines in the scheme of lausenite show

F igure 5. XRD data showing the hydration process of M-Fe 2 (SO 4 ) 3 at 53% RH. Several peaks were marked with the initials of corresponding mineral names, either followed by the index numbers in parentheses or not. F = Ferricopiapite; R = Rhomboclase; K = Kornelite; P = Paracoquimbite

Figure 6 Comparison of estimated and experimental data for reactions in the system Fe 2 (SO 4 ) 3 -H 2 O as a function of temperature and water activity. ( a ) This work and additive model ( Mercury et al. 2001 ); ( b ) system mikasaite-kornelite (7 H 2 O)-coquimbite; ( c ) system mikasaite-Fe 2

F igure 3. Polyhedral representation of the corrugated Fe 2 (SO 4 ) 3 (H 2 O) 5 slabs which build the structure of this phase, and the Fe 2 (SO 4 ) 3 (H 2 O) 6 slabs in the structure of kornelite. The octahedral-tetrahedral (O-T) chains and rows of sulfate tetrahedra are marked

. For example, the triangle at 60 days and 53% RH drops in the region of kornelite, but the sample at this stage still contains ferricopiapite and rhomboclase besides kornelite.

; K = Kornelite. It can be seen that rhomboclase crystallized at the beginning, but converted to kornelite after 60 h.

Fig. 3 Crystal structures of the lausenite ( Majzlan et al. , 2005 ), kornelite ( Robinson & Fang, 1973 ), dehydrated kaňkite (this work), and a hypothetical model of the possible kaňkite structure. The transformation between kornelite and lausenite is reversible ( Kong et al. , 2011

Figure 29. (a) The octahedral–tetrahedral sheet in the minerals of the alunite supergroup; (b) the structure of alunite; (c) the octahedral sheet in felsöbányaite; (d) the structure of felsöbányaite projected down [010]; (e) the [ M 2 ( T Ø 4 ) 3 Ø 6 ] sheet in kornelite; (f) the structure

F igure 1. Optical light microscopy images of kornelite ( a ) and paracoquimbite ( b ) crystals used for the thermodynamic measurements.

F igure 3. A polyhedral drawing of the kornelite structure projected parallel to c . Corrugated sheets (seen edge-wise) are formed by corner-sharing SO 4 tetrahedra and Fe 3+ (O,H 2 O) octahedra. Protons (H) are represented by small spheres. Dashed lines indicate hydrogen bonds. Channel H 2 O

F igure 4. A difference Fourier electron-density map of kornelite after removing channel H 2 O from the atom list. Channel H 2 O is smeared out parallel to the c -axis and displays two elongate thickenings. Solid lines represent 0.5 e/Å 3 contours, whereas dashed lines represent −0.5 e/Å 3 .

ferric sulfates. To display the equilibrium between kornelite and Fe 2 (SO 4 ) 3 ·5.03H 2 O, unrealistic relative humidities higher than 100% had to be plotted on the abscissa. ( b ) A phase diagram calculated with the same data as in a with the exception of S °(kornelite) and S °(paracoquimbite

Figure 5 ( a ) Mid-infrared thermal emissivity spectra of sulfates with sheets of SO 4 tetrahedra and MX 6 octahedra, and their crystal structure diagrams—including goldichite ( Graeber and Rosenzweig 1971 ), kornelite ( Robinson and Fang 1973 ), rhomboclase ( Mereiter 1974 ), slavikite

F igure 2. Results of differential thermal ( a ) and thermogravimetric ( b ) analysis of the two studied sulfates. The data for kornelite are shown by the solid curve, and those for paracoquimbite are shown by the dashed curve.

F igure 7. XRD data showing the hydration process of T-Fe 2 (SO 4 ) 3 at 53% RH. F = Ferricopiapite; R = Rhomboclase; K = Kornelite; P = Paracoquimbite; U = Unknown phase.