Single‐crystal X‐ray and electron‐diffraction studies show the existence in one polymorph of 4CaO.Al 2 O 3 . 13H 2 O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca 2 (OH) 7 ‐ 3H 2 O. These structural elements are stacked in a complex way and there are probably two or more poly‐types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1 2 O 3 .13H 2 O, from which it is derived by substitution of CO 3 2‐ for 20H ‐ + 3H 2 O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al 2 O 3 .Ca Y 2 ‐ xH 2 O and 3CaO Al 2 O 3 Ca Y xH 2 O. On dehydration, 4CaO.Al 2 O 3 .13H 2 O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH) 2 and 4CaO.3Al 2 O 3 .3H 2 O are formed and, by 1000°C., CaO and 12CaO.7Al 2 O 8 . The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al 2 O 3 .3H 2 O is formed.
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Buttler et al. (1959) studied this question.
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