Thermal expansion of alunite up to dehydroxylation and collapse of the crystal structure

Abstract

The high-temperature (HT) behaviour of a sample of natural alunite was investigated by means of in situ HT single-crystal X-ray diffraction from room temperature up to the dehydroxylation temperature and consequent collapse of the crystal structure. In the temperature range 25À500ºC, alunite expands anisotropically, with most of the contribution to volume dilatation being produced by expansion in the c direction. The thermal expansion coefficients determined over the temperature range investigated are: a a = 0.61(2)610 À5 K À1 (R 2 = 0.988), a c = 4.20(7)610 À5 K À1 (R 2 = 0.996), a c /a a = 6.89, a V = 5.45(7)610 À5 K À1 (R 2 = 0.998). At~275À300ºC, a minor discontinuity in the variation of unit-cell parameters with temperature is observed and interpreted on the basis of loss of H 3 O + that partially substitutes for K + at the monovalent A site in the alunite structure. Increasing temperature causes the Al(O,OH) 6 sheets, which remain almost unaltered along the basal plane, to move further apart, and this results in an expansion of the coordination polyhedron around the intercalated potassium cation. Sulfate tetrahedra act as nearly rigid units, they contract a little in the lower temperature range to accommodate the elongation of the Al octahedra.

Jarosite occurrence in the Deccan Volcanic Province of Kachchh, western India: Spectroscopic studies on a Martian analog locality

“…Higher in the sequence, sample A14, which represents the white‐ and light purple‐colored soft, powdery clay at Site A, contains alunite and natroalunite‐2c (Figure ). The diagnostic diffraction peaks used for identification of alunite are 5.68, 3.48, 2.97, and 1.89 Å [ Zema et al , ], whereas the peaks at 4.89, 2.21, 1.74, and 1.46 Å were matched with those for natroalunite‐2c [ Osaka et al , ]. Sample A15, from the cream yellow layer of the distinctly colored clay strata of the Matanumadh Formation, has d values of 5.91, 5.56, 5.04, 3.11, 3.05, 2.52, 1.97, and 1.82 Å (Figure ) that match the data for jarosite [ Basciano and Peterson , ].…”

Kinetics of Direct Leaching of Natural Alunite in KOH

“…The extent of this replacement depends on temperature and solution composition. [9][10][11][12][13][14] Natural alunites contain Al, K, and Na, which can be recovered as K 2 SO 4 , Na 2 SO 4 , Al 2 O 3 , Al 2 (SO 4 ) 3 , or potassium alum KAl(SO 4 ) 2 . 6 The main method of alunite processing and recovering its valuable products is hydrometallurgy.…”

“…A minor replacement of K and Na by H 3 O + is reported in natural alunite, but its magnitude is considerably higher in synthetic alunite. The extent of this replacement depends on temperature and solution composition. − Natural alunites contain Al, K, and Na, which can be recovered as K 2 SO 4 , Na 2 SO 4 , Al 2 O 3 , Al 2 (SO 4 ) 3 , or potassium alum KAl(SO 4 ) 2…”

Study of the dehydroxylation of kaolinite and alunite from a Mexican clay with DRIFTS-MS

“…These substitutions can be achieved from 400°C in the active sites generated via the loss of -OH{5, 6} groups, keeping kaolinite as a mineral phase. It could also induce partial or total substitutions of the sites generated by the loss of -OH{14} groups from 500°C, which would improve its mechanical properties via generation of metakaolinite (Li et al, 2010;Ferone et al, 2013;Peys et al, 2016) and decomposition of alunite (Frost et al, 2006;Zema et al, 2012) Therefore, the substitutions with gaseous metal oxides impart diverse mechanical properties for technological applications as catalytic support and building materials to the metakaolinite and the meta-alunite.…”

“…Alunite (KAl 3 (SO 4 ) 2 (OH) 6 .nH 2 O) consists of octahedral aluminium bonded to sulfate and potassium ions ( Fig. 2) (Kristóf et al, 2010;Zema et al, 2012). In its natural state, it contains structural and adsorbed water.…”