Montesommaite, (K,Na)₉Al₉Si₂₃O₆₄.10H₂O, a new zeolite related to merlinoite and the gismondine group

The new zeolite montesommaite, (K,Na)₉Al₉Si₂₃O₆₄ · 10H₂O, occurs with dolomite, calcite, chabazite, and natrolite in vesicles in scoria from Pollena, Monte Somma-Vesuvius, Italy. Montesommaite forms transparent, colorless, dipyramidal crystals, having a vitreous luster, a white streak, and no cleavage. The observed and calculated densities are 2.34 ± 0.04 and 2.30 g/cm³, respectively. Optically, it is biaxial negative, with 2V = 35 ± 5° (observed) and 39° (calculated), α = 1.498, β = 1.506, γ = 1.507 (all ±0.001), and no dispersion. The orientation is X = c, Y = a or b, and Z = a or b (a and b are indistinguishable). An electron microprobe analysis gave K₂O 16.7, Na₂O 0.2, Al₂O₃ 19.8, SiO₂ 55.7, H₂O 7.6 (by difference), sum 100.0 wt%. This yields K_8.8Na_0.2Al_9.7Si_23.1O_75.6H_21.0 or, ideally, (K,Na)₉Al₉Si₂₃O₆₄ · 10H₂O for the unit-cell contents. Montesommaite is orthorhom-bic, Fdd2, with a = b = 10.099(1), c = 17.307(3) Å, and Z = 1. The mineral is very nearly tetragonal, I42d or I4₁md (pseudo-I4₁/amd), with $A=a/2$ and C = c = 17.307 Å. The strongest powder X-ray diffraction lines (d_obs,I_obs,hkl) are 6.589(75)111; 4.334(43)004; 3.299(100)222; 3.130(100)311,115; 2.797(30)313; 2.513(18)400,206; 2.347(22)315; 1.784(22)440; 1.720(16)515,337; and 1.558(18)2·2·10. Similarities in cell parameters, symmetry, and chemical formulae strongly imply that montesommaite is related to merlinoite (K₅Ca₂Al₉Si₂₃O₆₄ · 24H₂O) and to the members of the gismondine group. This suggestion is verified by a determination of the idealized substructure of montesommaite in 14,/amd symmetry using powder diffraction intensities.