Mampsisite, Ca4Al2(OH)12(CO3)·5H2O, a new mineral and a new polymorph of the cementitious AFm monocarboaluminate

Abstract Mampsisite is a new mineral in the hydrocalumite group – a family of natural layered double hydroxides (LDH), the analogues of cementitious calcium aluminates, or AFm phases. The mineral is the first purely carbonate member of the hydrocalumite group. Mampsisite was discovered in peralkaline hydrothermal assemblages confined to pyrometamorphic rocks of the Hatrurim Basin in the Negev desert, nearby the Israeli coast of the Dead Sea. The mineral occurs in the cavities of marble-like larnite-brownmillerite-jasmundite rock, where it associates with katoite, portlandite and other natural Ca-Al LDH: kuzelite Ca4Al2(OH)12(SO4)·6H2O, hydrocalumite Ca4Al2(OH)12(Cl,CO3,OH)2·4H2O, and mariakrite Ca4Al2(OH)12(Fe2S4)·4H2O. The assemblage was formed from residual highly alkaline solutions produced by late hydrothermal alteration of larnite, Ca2SiO4, and jasmundite, Ca11(SiO4)4O2S. Mampsisite forms colorless platy crystals up to 50 μm across with a perfect cleavage on {001}. Calculated density is 2.180 g·cm−3. Chemical composition (electron microprobe, wt.%, water and CO2 calculated from structural data): CaO 39.12, Al2O3 17.66, CO2 7.66, H2O 34.49, total 98.93, corresponding to the empirical formula Ca4.01Al1.99(CO3)1.00(OH)11.99·5.00H2O or ideally Ca4Al2(OH)12(CO3)·5H2O. The mineral is triclinic, space group P1¯, a = 5.7834(2), b = 9.9274(3), c = 15.0972(4) Å, α = 87.198(2), β = 89.805(2), γ = 89.967(2) °, V = 865.75(5) Å3, and Z = 2. The crystal structure (R1 = 0.053) represents an ordered 1:1 interstratification of two kinds of fully ordered layered units: the positively charged, carbonate free hydrocalumite layer [Ca4Al2(OH)12(H2O)4]2+ and the AFm layer of previously unknown type – negatively charged [Ca4Al2(OH)12(H2O)2(CO3)2]2−. The counter-charged layers are separated by two H2O molecules per formula unit, which are lying in the interlayer. This arrangement is different from that of synthetic AFm monocarboaluminate, which is composed of the uniform neutral units [Ca4Al2(OH)12(H2O)3(CO3)]0. However, both layer arrangements yield the same bulk chemical formula, Ca4Al2(OH)12(CO3)·5H2O. The strongest lines of X-ray powder diffraction pattern of mampsisite [d in Å(I)(hkl)]: 7.58(100)(002), 3.774(37)(004), 2.845(11)(131), 2.724(14)(1¯32), 2.514(20)(203), 2.451(19)(1¯3¯3), 2.330(14)(1¯34). It should be emphasized that basal interlayer spacing of mampsisite is almost identical to that of synthetic AFm monocarboaluminate and to basal spacings of 2:1 layered double hydroxides – quintinite, Mg4Al2(OH)12(CO3)·3H2O, and the members of the quintinite group. The latter implies that mampsisite may be relatively common but a hidden constituent of cementitious materials and natural Ca-bearing clays, being readily misidentified with another polymorph of AFm monocarboaluminate and quintinite during the routine powder XRD analyses.