The Cause of Anisotropy in Symmetrically Cubic Almandine and Spessartine Garnet
Creators: name: Round, Stephanie
Publisher: name: Graduate Studies
Year: 2013
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Mineralogy
subject: Inorganic
subject: garnet
subject: Anisotropy
subject: Spessartine
subject: Almandine
subject: birefringence

description: The origin of optical anisotropy in garnet has been an unanswered question for over a century. Previous studies have hypothesized birefringence to be related to large scale twinning, the inclusion of OH groups within the tetrahedral site, cation ordering of the dodecahedral and octahedral sites and strain leading to cubic symmetry violations. This thesis addresses previous hypotheses and suggests a new solution to the problem of anisotropy in almandine and spessartine garnet; the presence of multiple structural intergrowths within the crystal structure causing strain, resulting from a mismatch within the structure at the boundary of each phase. This hypothesis satisfies the possible presence of optical birefringence while maintaining a cubic symmetry of space group Ia d. Four samples were studied: one almandine and three spessartine garnets. The techniques used to solve the above mentioned question include electron microprobe analysis (EMPA) and synchrotron high-resolution powder X-ray diffraction (HRPXRD). The composition and general formula of each sample was obtained from EMPA analyses. The almandine from Tanzania is Alm56Prp23Sps9Grs7, {Fe1.68Mg0.70Ca0.29Mn0.28}Σ=2.95[Al1.92Fe0.14]Σ=2.06(Si2.94Al0.06)Σ=3.00O12, the spessartine from Loliondo, Tanzania is Sps60Prp27, {Mn1.97Mg0.82Ca0.11}Σ=2.90[Al1.89Mn0.12Fe0.08]Σ=2.10(Si2.90Al0.11)Σ=3.00O12, the spessartine from Colorado, USA is Sps73Alm18, {Mn2.20Fe0.60Mg0.05Ca0.03}Σ=2.87[Al1.86Fe0.24Ti0.02]Σ=2.13(Si2.85Al0.15)Σ=3.00O12, and the spessartine from California, USA is Sps90Alm8, {Mn2.70Fe0.24}Σ=2.94[Al1.98Mn0.08Ti0.01]Σ=2.06(Si2.94Al0.06)Σ=3.00O12. The results gathered from HRPXRD indicate the presence of two structural intergrowths within each of the four samples, results of the most dominant phase for each are as follows: almandine from Tanzania χ2 = 2.341, R(F2) = 0.0337, a = 11.56114(2)Å, weight % = 54.4(2), sof X(Fe) = 0.769(2), Y(Al) = 0.917(3), Y(Si) = 0.875(3), average = 2.3085Å, Y-O = 1.894(2)Å, and Z-O = 1.6366(9)Å; spessartine from Loliondo, Tanzania has χ2 = 4.114, R(F2) = 0.0466, a = 11.59829(2)Å, weight % = 62.8(2), sof X(Mn) = 0.802(2), Y(Al) = 0.937(3), Z(Si) = 0.918(3), average = 2.3207Å, Y-O = 1.898(2)Å, and Z-O = 1.638(2)Å; spessartine from Ruby Mountain, Colorado, USA has χ2 = 2.997, R(F2) = 0.0888, a = 11.60696(2)Å, weight % = 97.5(1), sof X(Mn) = 0.949(3), Y(Al) = 0.965(4), Z(Si) = 0.916(3), average = 2.3212Å, Y-O = 1.895(2)Å, and Z-O = 1.643(2)Å; spessartine from Ramona, San Diego County, California, USA has χ2 = 1.303, R(F2) = 0.0367, a = 11.61331(6)Å, weight % = 62.12(8), sof X(Mn) = 0.942(1), Y(Al) = 0.936(2), Z(Si) = 0.919(2), average = 2.3257Å, Y-O = 1.8976(6)Å, and Z-O = 1.6395(6)Å. Each of the four samples contain two cubic structural intergrowths that have differing structural parameters, the mismatch between the two structural phases causes strain within the crystal and results in optical birefringence of symmetrically cubic garnet minerals.