Evolution of fumarolic anhydrous copper sulfate minerals during successive hydration/dehydration

Abstract

Hydration processes of primary anhydrous minerals as well as dehydration of the hydrated phases are relevant not only for answering geochemical and petrological questions, but are also interesting in the context of the theory of the ‘Evolution of minerals’. Our study of the evolution of anhydrous exhalative sulfates in hydration and dehydration processes has demonstrated the complexity of the processes for a number of minerals from the active high-temperature fumaroles of Tolbachik volcano (chalcocyanite Cu(SO4), dolerophanite Cu2O(SO4), alumoklyuchevskite K3Cu3AlO2(SO4)4 and itelmenite Na2CuMg2(SO4)4). Hydration and dehydration experiments were carried out for all four minerals using powder X-ray diffraction. A typical structural characteristic of several anhydrous copper sulfate minerals of fumarolic origin is the presence of oxygen-centred OCu4 tetrahedra. These are absent in the structures of all known hydrated minerals or synthetic compounds of the class under consideration. Hydration of minerals initially containing O2– anions as part of oxocomplexes, proceeds with sequential formation of a large series of hydroxysalts. On the contrary, hydration of itelmenite with its relatively complex ‘initial’ structure, but without additional oxygen atoms that are strong Lewis bases, results in formation of simpler hydrates. The lower the temperature and the larger the excess of water, the stronger the tendency of the cations to adopt higher hydration numbers thus outcompeting the sulfate anions as ligands. Ultimately, the water molecules completely expel the bridging sulfate anions from the metal coordination sphere yielding relatively simple fully hydrated structures.

Anhydrous alkali copper sulfates – a promising playground for new Cu2+ oxide complexes: new Rb-analogues of fumarolic minerals.

“…upon hydration-dehydration cycles. Yet, our studies of such processes for selected copper sulfate minerals indicate that these are very complicated (Siidra et al, 2021a), include numerous steps and are not always reversible even for initially single-phase specimens. It is however very likely that these studies, extended into more complex systems, will produce even more intricate and intriguing architectures, compositions and possibly properties.…”

“…The obtained new compound shows the probability of detecting such compounds (especially for copper-containing ones) in the zones of fumaroles exposed to intense atmospheric precipitation. Alkali copper sulfates are known to form multiple hydrated "offsprings" during exposure to moist air (Siidra et al, 2021b).…”

“…High hygroscopicity and rapid hydration was reported for several potassium analogues, e.g. K 2 Cu(SO 4 ) 2 (Zhou et al, 2020) and K 2 Cu 2 (SO 4 ) 3 (Lander et al, 2017); most natural copper sulfate minerals are also unstable in moist air (Siidra et al, 2019a(Siidra et al, , 2021a. The most likely reason for such behaviour is non-rigidity of the coordination sphere of both alkali cations (due to weak binding to any ligands) and copper (due to the pronounced Jahn-Teller effect).…”

Morphotropism in fumarolic mineral-related anhydrous sulfates: novel representatives in A + 2 M 2+(SO4)2 and A + 2 M 2+ 2(SO4)3 series

“…Testasicca et al (2016)] but with little success. During the last three years, we have found that A 2 Cu(SO 4 ) 2 (A = Na, K, Rb) compounds constitute a complex morphotropic series which consists of at least five different structural modifications [-, -,and -phases, see Siidra et al (2021a)]. This work completes the structural series from the large-cation side adding the "-type for A = Cs (Table 5).…”

“…It is worth noting that most known alkali copper sulfate representatives are structurally based on threedimensional copper sulfate frameworks (Borisov et al, 2021). ISSN 2052-5206 While several examples of Rb-Cu sulfates have been recently reported (Siidra et al, 2021a), anhydrous sulfates of caesium with copper, as well as with cobalt, remain mostly unaddressed; to the best of our knowledge, no reliable data have been reported. In order to fill this gap and to elucidate the effect of alkali cation size on the structural architectures of two relatively numerous series, A I 2 Cu(SO 4 ) 2 and A I 2 M II 2 (SO 4 ) 3 , we have undertaken this study.…”