The crystal structures of the minerals in the cuspidine-wohleritc- lavenite group can be described in terms of two kinds of modules whose connections form the structures of the various phases: "octahedral walls" four columns wide and running along c, and diorthosilicate groups. The octahedral walls are interconnected by corner sharing to create a framework that is the common feature in all the structuresof the family. There are various waysto distrib ute the Si207 units, whichserveto strengthen the frame work. Ten different structure-types were derived, within fixed cell dimensions (a = b = 10.5 A, c = 7.3 A). One of these structure-types, with topological symmetry P21/a, is shown by cuspidine and lavenite, both with space-group symmetry Pl^/a, as well as by niocalite, in which the space-group symmetryis lowered to Pa by the ordering of the octahedral cations. Another structure-type, with topological symmetry P-l, is shown by wohlerite.The enumeration of the possible structure-types wassuccessfully applied to determine the unknown structures ofother natural phases in the group: hiortdahlite I and II, baghdadite, and a natural phase from the Burpala Massif, USSR. The disorder phenomena commonly occurring in these minerals, such as domain structure, micro-scale twinning, and stacking faults, can be interpreted on the basis of the "modular" structure.
Modular mineralogy in the cuspidine group of minerals
MERLINO, STEFANO;PERCHIAZZI, NATALE
1988-01-01
Abstract
The crystal structures of the minerals in the cuspidine-wohleritc- lavenite group can be described in terms of two kinds of modules whose connections form the structures of the various phases: "octahedral walls" four columns wide and running along c, and diorthosilicate groups. The octahedral walls are interconnected by corner sharing to create a framework that is the common feature in all the structuresof the family. There are various waysto distrib ute the Si207 units, whichserveto strengthen the frame work. Ten different structure-types were derived, within fixed cell dimensions (a = b = 10.5 A, c = 7.3 A). One of these structure-types, with topological symmetry P21/a, is shown by cuspidine and lavenite, both with space-group symmetry Pl^/a, as well as by niocalite, in which the space-group symmetryis lowered to Pa by the ordering of the octahedral cations. Another structure-type, with topological symmetry P-l, is shown by wohlerite.The enumeration of the possible structure-types wassuccessfully applied to determine the unknown structures ofother natural phases in the group: hiortdahlite I and II, baghdadite, and a natural phase from the Burpala Massif, USSR. The disorder phenomena commonly occurring in these minerals, such as domain structure, micro-scale twinning, and stacking faults, can be interpreted on the basis of the "modular" structure.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
