THE CRYSTAL CHEMISTRY OF THE SCAPOLITE-GROUP MINERALS. II. THE ORIGIN OF THE I4/m P42/n PHASE TRANSITION AND THE NONLINEAR VARIATIONS IN CHEMICAL COMPOSITION

Abstract

The scapolite structure consists of two interpenetrating frameworks: [(Si,Al)12O24] and {Na,Ca}{Cl,(CO3)}. 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 (CO3). The short-range bond-valence requirements of the (CO3) group are satisfied by coordination with Ca4 at ~ Me 100, NaCa3 at ~ Me 80, Na2Ca2 at ~ Me 55, Na3Ca at ~ Me 40, and Na4 at ~ Me 20. The driving mechanism for the I 4/ m → P 42/ n phase-transitions is the coupling of SRO between the [(Si,Al)12O24] and {Na,Ca}{Cl,(CO3)} frameworks. At a composition of ~ Me 50, local bond-valence requirements force the local clusters Na2Ca2 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 I 4/ m symmetry and gives rise to two distinct tetrahedra, T (2) and T (3), dominated by Al and Si, respectively. The result is the P 42/ n scapolite structure. As Na or Ca become dominant, the number of Na2Ca2 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 ~ Me 22 and Me 78, the clusters NaCa3 and Na3Ca dominate, the driving mechanism for lower symmetry vanishes, and the structures revert to maximal I 4/ 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 (CO3) in end-member meionite]. Local bond-valence requirements indicate extensive SRO about both Cl and (CO3) in scapolite. In particular, Cl cannot occur in I 4/ m meionite except where K is present; in this case, chemical variations indicate that Cl enters the I 4/ m meionite structure as {ClK2Ca2} clusters.