Structural intergrowth of merlinoite/phillipsite and its temperature-dependent dehydration behaviour: a single-crystal X-ray study

Supposed 'merlinoite' crystals from Monte Somma, Vesuvius (Italy) and Fosso Attici, north of Rome (Italy) represent highly twinned coherent intergrowths between merlinoite and phillipsite on a submicroscopic level. The MER (Immm, a approximate to 14.1, b approximate to 14.2, c approximate to 9.9 angstrom) and PHI (P2(1)/m, a approximate to 9.9, b approximate to 14.3, c approximate to 8.7 angstrom, beta = 124.8 degrees) frameworks of similar composition are assembled from identical tetrahedral units, though with a different connectivity. Coherent intergrowth and twinning of the two frameworks lead to P4(2)/mnm pseudosymmetry, which is diagnostic of the intergrowth. Under ambient conditions merlinoite has Immm symmetry or I4/mmm if twinned. A low-symmetry model of space group P12(1)/m1 (a approximate to 14.2, b approximate to 14.2, c approximate to 10 angstrom, beta = 90 degrees) allows structure refinement and quantification of the two frameworks. Upon in situ dehydration to 250 degrees C the evolution of the unit-cell volume of the Monte Somma merlinoite/phillipsite intergrowth displays an intermediate trend between previously studied pure merlinoite from the Khibiny massif (Russia) and Ba-rich phillipsite. The Monte Somma crystal studied by temperature-dependent single-crystal X-ray diffraction methods also contained a subordinate chabazite inclusion with no coherent structural relationship to the merlinoite/phillipsite framework. Thus, the modification of the chabazite framework on dehydration could also be studied.