Topotactic transformation and dehydration of the zeolite gismondine to a novel Ca feldspar structure

Abstract

Temperature-dependent single-crystal X-ray data were collected on gismondine ( GIS ) Ca 4 (Al 8 Si 8 O 32 )·18H 2 O from Rio Pian del Foco, Genova province, Italy, in steps of 25 °C up to 600 °C. At room temperature, gismondine has space group P 2 1 / c with a = 10.0214(1), b = 10.5997(1), c = 9.8327(1) A, β = 92.363(1)°, V = 1043.58(2) A 3 . This structure remained stable up to 50 °C. The dehydration behavior then divided into two different pathways depending on the sample. In the more frequent path I, the LT P 2 1 2 1 2 1 structure (phase B) Ca 4 (Al 8 Si 8 O 32 )·12H 2 O [ a = 13.6801(8), b = 10.4670(6), c = 13.8667(9) A, V = 1985.6(2) A 3 ] formed at 75 °C. The orthorhombic structure has a doubled volume relative to the monoclinic room-temperature structure. At 150 °C the HT P 2 1 2 1 2 1 structure (phase C) with 8 H 2 O pfu [ a = 13.9014(12), b = 8.9469(8), c = 13.9697(14) A, V = 1737.5(3) A 3 ] occurred. This phase C has strongly compressed elliptical channels with Ca ions bonding to adjacent walls. At high temperature (300 °C), the quality of the diffraction pattern in path I further degraded and became inclusive. In path II the diffraction patterns were of considerably higher quality and at 75 °C the phase LT I 2/ a with 16 H 2 O pfu [ a = 9.790(2), b = 10.437(2), c = 9.790(2) A, β = 90.97(3)°, V = 1000.1(4) A 3 ] formed, changing at 150 °C to HT I 2/ a [at 225 °C: a = 9.434(4), b = 9.044(2) c = 9.695(2), β = 89.04(1)°, V = 827.0(4) A 3 ] with 4 H 2 O. Above 250 °C the HT I 2/ a structure topotactically transformed by a reconstructive mechanism to a triclinic C 1 Ca feldspar structure [ a = 8.152(5), b = 12.917(5), c = 7.126(4) A, α = 93.26(3), β = 116.37(6), γ = 88.72(5)°, V = 671.2(7) A 3 ], which does not follow Loewenstein’s (1954) rule, as the framework has ordered corner-linked AlO 4 tetrahedra. As a consequence of the GIS to Ca feldspar transformation T-O bonds within four-membered rings break and reconnect to a new framework type. The HT I 2/ a structure with strongly twisted double crankshaft chains acts as precursor for the feldspar formation without an intermediate X-ray amorphous phase usually found after complete dehydration of most natural zeolites. This study reports for the first time a low-temperature topotactic transformation from gismondine to Ca feldspar and explains the highly unusual occurrence of ordered Al-O-Al clusters in this feldspar structure.