A Modular Concept of Crystal Structure Applied to the Thermal Transformation of α‐ C 2 SH

A single crystal of α‐ Ca 2 [ HSiO 4 ]( OH ) (α‐ C 2 SH ) was repeatedly imaged at room temperature with synchrotron mid‐infrared microscopy after heating to 310°C, 340°C, 370°C, and 400°C respectively. The mechanisms of the observed phase transformations are discussed on the basis of a modular concept of the crystal structures. All images show domains of dellaite, Ca 6 [ Si 2 O 7 ][ SiO 4 ]( OH ) 2 , which are predominantly formed in the core of the crystal. In the crystal rim area α‐ C 2 SH persists in higher abundance. The mechanism of the phase transformation of α‐ C 2 SH into dellaite includes the following: (1) Partial formation of killalaite ( Ca 3 [ HSi 2 O 7 ]( OH )) as nuclei according to the isochemical reaction 2 Ca 2 [ HSiO 4 ]( OH ) → Ca 3 [ HSi 2 O 7 ]( OH ) +  Ca ( OH ) 2 probably induced by anisotropic thermal expansion, local chemical fluctuations, structural (proton) disorder, and different bond strengths of the OH groups in the α‐ C 2 SH structure. (2) Further dehydration of killalaite and α‐ C 2 SH domains results in the formation of dellaite according to Ca 3 [ HSi 2 O 7 ]( OH ) + Ca( OH ) 2  + Ca 2 [ HSiO 4 ]( OH ) – 2 H 2 O → Ca 6 [ Si 2 O 7 ][ SiO 4 ]( OH ) 2 . The results suggest that the polymerization of two isolated [ HSiO 4 ] tetrahedra takes place without dehydration according to reaction (1) rather than through condensation with simultaneous H 2 O release: 2[ HSiO 4 ] → [ Si 2 O 7 ] +  H 2 O . We suggest that reaction (1) cannot be completed at ambient pressure. Thus in the regions close to the rim of the crystals we expect the formation of x ‐ C 2 S , which starts along the crystal edges according to Ca 2 [ HSiO 4 ]( OH ) → Ca 2 SiO 4  +  H 2 O . Based on a modular concept, a structural relationship between α‐ C 2 SH , killalaite, dellaite, and x ‐ C 2 S has been established. Similarities and differences in the thermal behavior of α‐ C 2 SH and afwillite have been highlighted.