The origin of birefringence in near end-member andradite garnet
Creators: name: Klincker, Allison
Publisher: name: Graduate Studies
Year: 2013
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Mineralogy
subject: andradite
subject: garnet
subject: birefringence
subject: Mineralogy
subject: crystal structure
subject: synchrotron high-resolution powder X-ray diffraction (HRPXRD)
subject: single crystal X-ray diffraction (SXTL)

description: Birefringence in some cubic silicate garnet was observed over 100 years ago, but the origin of this anisotropy is still not known. This study examines four birefringent andradite garnet samples (ideally, Ca2Fe3Si2O12) from Crowsnest Pass, Alberta (CP-1), Graham County, Arizona (GCA-1 and GCA-2), and Ambanja, Madagascar (AM-1) using synchrotron high-resolution powder X-ray diffraction (HRPXRD), electron microprobe analysis (EMPA), and single crystal X-ray diffraction (SXTL) to understand the observed anisotropy or birefringence. The average chemical analyses for the four samples, in the general formula [8]X3[6]Y2[4]Z3[4]O12, are as follows: CP-1: {Ca_2.85 Mn_0.06^(2+) Fe_0.05^(2+) Mg_0.04 }_(Σ=3.00) [Fe_1.27^(3+) Al_0.47 Ti_0.19^(4+) Fe_0.07^(2+) ]_(Σ=2.00) (Si_2.88 Al_0.12 )_(Σ=3.00) O_12, Adr_64 Grs_19 Mrm_7; GCA-1: {Ca_3.04 }_(Σ=3.04) [Fe_1.90^(3+) Al_0.04 Mn_0.02^(3+) Mg_0.01]_(Σ=1.96) (Si_2.92 Al_0.04 Fe_0.04^(3+))_(Σ=3.00) O_12, Adr_95; AM-1: {Ca_3.04 }_(Σ=3.04) [Fe_1.95^(3+) Mg_0.02]_(Σ=1.97) (Si_2.95 Al_0.04 Fe_0.01^(3+))_(Σ=3.00) O_12, Adr_97; GCA-2: {Ca_2.99 Mg_0.01 }_(Σ=3.00) [Fe_1.99^(3+) Mn_0.01^(3+)]_(Σ=2.00) (Si_2.95 Al_0.03 Fe_0.02^(3+))_(Σ=3.00) O_12, Adr_98. HRPXRD data were analyzed using the Rietveld method and space group Ia3 ̅d. Three phases were observed in the HRPXRD trace of each sample. The R(F)2 value, a cell parameter, and weight percent (wt. %) of each phase, for each sample are as follows. CP-1: R(F)2 = 0.0315, for phase-1 a = 12.00006(2) Å, 62.85(7) wt. %; for phase-2 a = 12.04951(2) Å, 19.14(9) wt. %; for phase-3 a = 12.01968(3) Å, 18.0(1) wt. %. GCA-1: R(F)2 = 0.0291, for phase-1 a = 12.06314(1) Å, 51.9(3) wt. %; for phase-2 a = 11.9931(1) Å, 14.15(6) wt. %; for phase-3 a = 12.0564(1) Å, 33.9(3) wt. %. AM-1: R(F)2 = 0.0231, for phase-1 a = 12.062764(6) Å, 52.21(8) wt. %; for phase-2 a = 12.00599(2) Å, 9.26(8) wt. %; for phase-3 a = 12.05647(3) Å, 38.5(1) wt. %. GCA-2: R(F)2 = 0.0308, for phase-1 a = 12.05416(2) Å, 71.52(7) wt. %; for phase-2 a = 12.048854(5) Å, 24.5(1) wt. %; for phase-3 a = 12.06868(2) Å, 3.98(8) wt. %. SXTL results of samples CP-1 and GCA-1 indicate: a = 11.9930(9) Å, R1 = 0.0331, and wR2 = 0.0469 for the CP-1 sample, and a = 12.0510(7) Å, R1 = 0.0187, and wR2 = 0.0320 for the GCA-1 sample. The refinement results show that the HRPXRD method provides superior data and has the ability to observe multiple phases, both of which the SXTL method lacks. For the dominant phase in each sample, the following bond distances and site occupancy factor (sof) were observed: for CP-1, = 2.4196, Y-O = 2.831(1), and Z-O = 2.765(1) Å; sof, X = 0.970(2), Y = 0.763(1), and Z = 0.954(2). For GCA-1, = 2.4348, Y-O = 2.819 (1), and Z-O = 2.755(1) Å; sof, X = 0.955(2), Y = 0.930(2), and Z = 0.917(2). For AM-1, = 2.4319, Y-O = 2.884(0), and Z-O = 2.748(1) Å; sof, X = 0.955(2), Y = 0.941(2), and Z = 0.939(2). For GCA-2, = 2.4339, Y-O = 2.753(1), and Z-O = 2.753(1) Å; sof, X = 0.939(2), Y = 0.901(2), and Z = 0.950(2). The multiple cubic phases cause strain due to mismatch of the different cubic cells at the boundaries between each phase and give rise to the birefringence observed in each sample. This is the first study to show that birefringent garnet samples consist of multiple cubic phases.