Crystal chemistry of birefringent spessartine

摘要

The crystal structure of one isotropic [(1) Brazil] and three birefringent spessartine samples [(2) California, (3) Tanzania, and (4) Colorado] were refined using the Rietveld method, cubic space group Ia3-bard, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The results of electron-microprobe analysis (EMPA) indicate homogeneous compositions, in terms of end-members, as follows: (1) Sps_(54)Alm_(43), (2) Sps_(90)Alm_8, (3) Sps_(64)Prp_(27)Grs_3, and (4) Sps_(73)Alm_(19). Their crystal structures were modeled well as indicated by the Rietveld refinement statistics where the reduced x~2 and overall R (F~2) values for each sample are: (1) 1.395 and 0.0329, (2) 1.082 and 0.0354, (3) 1.025 and 0.0347, and (4) 1.016 and 0.0413. Two cubic phases occur in samples 2-4, and a single cubic phase occurs in sample-1. The dominant cubic phase-1 with locality, weight fraction (%), unit-cell parameter (A), distances (A), and site occupancy factors (sofs) are as follows: (1) Brazil: 100%, a = 11.581 54 (1), average = 2.3156, Al-O= 1.8949 (3), Si-O= 1.6376 (3) A, Mn(sof) = 0.961(1), Al(sof) = 0.945(1), and Si(sof) = 0.936(1); (2) California: 96.67(7)%, a = 11.613 32(1), average = 2.3249, Al-O=1.8956 (4), Si-O= 1.6416 (4) A, Mn(sof) = 0.951(1), M(sof) = 0.946(1), and Si(sof) = 0.927(1); (3) Tanzania: 69.46(6)%, a = 11.598 45(1), average = 2.3199, Al-O = 1.8964 (5), Si-0 = 1.6398 (5) A, Mn(sof) = 0.808(1), Al(sof) = 0.942 (1), and Si(sof) = 0.922(1); and (4) Colorado: 98.58(6)%, a= 11.606 89(1), average = 2.3204, Al-0 =1.8948 (6), Si-0 =1.6450 (6) A, Mn(sof) = 0.949(1), Al(sof) = 0.967(2), and Si (sof) = 0.913(2). The two-phase intergrowth causes strain that arises from mismatch of the structural parameters and gives rise to strain-induced birefringence.