THE CRYSTAL CHEMISTRY OF THE SCAPOLITE-GROUP MINERALS--II. THE ORIGIN OF THE I4/m reversible P4_2 PHASE TRANSITION AND THE NONLINEAR VARIATIONS IN CHEMICAL COMPOSITION

摘要

The scapolite structure consists of two interpenetrating frameworks: [(Si,Al)_(12)O_(24)] and {Na,Ca}{Cl,(CO_3)}. The unstrained dimensions of each framework are significantly different (as determined by the valence-matching principle and distance-least-squares refinement); there is an intrinsic steric stress between the two frameworks, the magnitude of which is a function of chemical composition. Evidence of the resulting structural strain is the large U_(eq) values for Cl at the A site across the marialite--meionite series and the large U_(eq) values for (Na,Ca) at the M site in marialite. Local bond-valence considerations indicate extensive SRO (Short-Range Order) about both Cl and (CO_3). The short-range bond-valence requirements of the (CO_3) group are satisfied by coordination with Ca_4 at approx Me_(100), NaCa_3 at approx Me_(80), Na_2Ca_2 at approx Me_(55), Na_3Ca at approx Me_(40), and Na_4 at approx Me_(20). The driving mechanism for the I4/m -> P4_2 phase-transitions is the coupling of SRO between the [(Si,Al)_(12)O_(24)] and {Na,Ca}{Cl,(CO_3)} frameworks. At a composition of approx Me_(50), local bond-valence requirements force the local clusters Na_2Ca_2 to occur in a trans configuration {NaCaNaCa} around the A site. Anion bond-valence requirements also force these clusters to link to each other through Na-O-Na or Ca-O-Ca linkages, and hence the combination of SRO and bond-valence requirements gives rise to LRO (long-range order). In turn, Al and Si are constrained to order at different tetrahedra, an ordering that breaks the topological I4/m symmetry and gives rise to two distinct tetrahedra, T(2) and T(3), dominated by Al and Si, respectively. The result is the P4_2 scapolite structure. As Na or Ca become dominant, the number of Na_2Ca_2 clusters decreases, the effectiveness of the symmetry-lowering mechanism lessens, and the degree of order of Al and Si over the T(2) and T(3) sites decreases toward both the marialite and meionite ends of the series. At the compositions approx Me_(22) and Me_(78) , the clusters NaCa_3 and Na_3Ca dominate, the driving mechanism for lower symmetry vanishes, and the structures revert to maximal I4/m symmetry. The nonlinear variations in bulk composition across the marialite--meionite series are also the result of SRO of M-site Na and Ca around the A site [which is occupied by Cl in end-member marialite and (CO_3) in end-member meionite]. Local bond-valence requirements indicate extensive SRO about both Cl and (CO_3) in scapolite. In particular, Cl cannot occur in I4/ m meionite except where K is present; in this case, chemical variations indicate that Cl enters the I4/m meionite structure as {ClK_2Ca_2} clusters.