Temperature-dependent single-crystal X-ray data were collected on gismondine (GIS) Ca4(Al8Si8O32)·18H2O from Rio Pian del Foco, Genova province, Italy, in steps of 25 °C up to 600 °C. At room temperature, gismondine has space group P21/c with a = 10.0214(1), b = 10.5997(1), c = 9.8327(1) Å, β = 92.363(1)°, V = 1043.58(2) Å3. 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 P212121 structure (phase B) Ca4(Al8Si8O32)·12H2O [a = 13.6801(8), b = 10.4670(6), c = 13.8667(9) Å, V = 1985.6(2) Å3] formed at 75 °C. The orthorhombic structure has a doubled volume relative to the monoclinic room-temperature structure. At 150 °C the HT P212121 structure (phase C) with 8 H2O pfu [a = 13.9014(12), b = 8.9469(8), c = 13.9697(14) Å, V = 1737.5(3) Å3] 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 I2/a with 16 H2O pfu [a = 9.790(2), b = 10.437(2), c = 9.790(2) Å, β = 90.97(3)°, V = 1000.1(4) Å3] formed, changing at 150 °C to HT I2/a [at 225 °C: a = 9.434(4), b = 9.044(2) c = 9.695(2), β = 89.04(1)°, V = 827.0(4) Å3] with 4 H2O. Above 250 °C the HT I2/a structure topotactically transformed by a reconstructive mechanism to a triclinic C1̄ Ca feldspar structure [a = 8.152(5), b = 12.917(5), c = 7.126(4) Å, α = 93.26(3), β = 116.37(6), γ = 88.72(5)°, V = 671.2(7) Å3], which does not follow Loewenstein’s (1954) rule, as the framework has ordered corner-linked AlO4 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 I2/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.

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