Parádsasvárite, a new member of the malachite-rosasite group from Parádsasvár, Mátra Mountains, Hungary

Ab Initio Thermodynamics and the Relationship between Octahedral Distortion, Lattice Structure, and Proton Substitution Defects in Malachite/Rosasite Group Endmember Pokrovskite Mg2CO3(OH)2

“…Pokrovskite is more stable than both brucite and MgO, indicating that any pokrovskite that forms kinetically under the water-saturated scCO 2 conditions will be capable of sequestering carbon successfully even if it is not able to transform to hydromagnesite or magnesite. The AIT results indicating the stability of pokrovskite under CO 2 -rich conditions are consistent with geological observations of likely formation conditions of cold carbonate-rich subsurface water for chukanovite and parádsasvárite …”

“…There is considerable evidence for water defects in other members of the malachite/rosasite group. IR spectra of parádsasvárite by Fehér identified a low-intensity band at 1637 cm –1 that was ascribed to molecular water adsorbed on the surface . A weak, well-separated band was also observed at 3647 cm –1 that was tentatively assigned to weakly hydrogen-bonded surface water but may be due to internal water formation from the Mg/2H substitutions.…”

“…More recent structural studies by Perchiazzi and Merlino confirmed the cation deficiency with an analysis of Mg 1.77 CO 3 (OH) 1.54 ·0.46H 2 O in their sample from Sonoma County, California . There is some evidence that this M n + / n H + substitution may be a more general phenomenon, particularly for the non-Cu containing members of the rosasite family such as chukanovite (Fe), nullaginite (Ni), and parádsasvárite (Zn), though it is known formally for complete stoichiometry only for copper uranium oxide CuU 3 O 10 /H 2 U 3 O 10 …”

“…The vast majority of malachite/rosasite minerals are formed from divalent transition metal cations Cu, Zn, Ni, Co, Ni, and Fe. Magnesium is the only group II metal identified to crystallize in the malachite/rosasite structure, ,, forming pokrovskite (Mg 2 CO 3 (OH) 2 ) and mcguinnessite (CuMgCO 3 (OH) 2 ) . This raises the question as to why pokrovskite and all other nonpredominantly Cu divalent basic carbonates with M 2 CO 3 (OH) 2 stoichiometry crystallize in the rosasite instead of the malachite structure.…”

“…Girgsdies and Behrens have hypothesized that the rosasite group minerals may be more widespread in nature than previously thought but are underreported due to their fibrous boitroydal nature . Indeed, new members of the group are being continuously discovered, such as parádsasvárite Zn 2 CO 3 (OH) 2 , by Fehér and co-workers in 2015 . M 2 CO 3 (OH) 2 minerals, particularly pokrovskite, may be relevant for carbon sequestration, as indications are it and other rosasites form in CO 2 -rich environments .…”