A single crystal of alpha-Ca-2[HSiO4](OH) (alpha-C2SH) was repeatedly imaged at room temperature with synchrotron mid- infrared microscopy after heating to 310 degrees C, 340 degrees C, 370 degrees C, and 400 degrees 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[Si2O7][SiO4](OH)(2), which are predominantly formed in the core of the crystal. In the crystal rim area alpha-C2SH persists in higher abundance. The mechanism of the phase transformation of alpha-C2SH into dellaite includes the following: (1) Partial formation of killalaite (Ca-3[HSi2O7](OH)) as nuclei according to the isochemical reaction 2Ca(2)[HSiO4] (OH) -> Ca-3[HSi2O7](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 alpha-C2SH structure. (2) Further dehydration of killalaite and alpha-C2SH domains results in the formation of dellaite according to Ca-3[HSi2O7](OH) + Ca(OH)(2) + Ca-2[H-SiO4](OH) - 2H(2)O -> Ca-6[Si2O7][SiO4](OH)(2). The results suggest that the polymerization of two isolated [HSiO4] tetrahedra takes place without dehydration according to reaction (1) rather than through condensation with simultaneous H2O release: 2[HSiO4] -> [Si2O7] + H2O. 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-C2S, which starts along the crystal edges according to Ca-2[HSiO4](OH) -> Ca2SiO4 + H2O. Based on a modular concept, a structural relationship between alpha-C2SH, killalaite, dellaite, and x-C2S has been established. Similarities and differences in the thermal behavior of alpha-C2SH and afwillite have been highlighted.