Triclinic and monoclinic phases. The pentaborate polyanion [B 5 O 12 ] (super 9-) , consisting of three borate tetrahedra and two borate triangles, can exist in two stereoisomeric configurations(l or d). These polyanions, by sharing tetrahedral corners, form chains parallel to the c axis, which are connected to four such adjacent chains along the a and b axis, such that the corners of borate tetrahedra share corners of borate triangles and vice versa. A pseudo-tetragonal open borate framework structure is formed. Borate chains composed of pentaborate polyanions in l or d configurations are linked within the structure. The term "polytropism" is suggested for this particular form of polymorphism. Water molecules are held by weak calcium-oxygen bonds and O-H...Cl type hydrogen bonds; hence, they are zeolitic in nature. Substitution of the chlorine atoms by hydroxyl ions gives rise to the tyretskite group of minerals.--Modified journal abstract.