Anomalous optical variations in the grossular garnet from the Eden Mills,Belvidere Mountain (Vermont,USA)

The optical anomalies, and surface and lamellar textures of a birefringent grossular garnet crystal from the Eden Mills, Belvidere Mountain, Vermont, USA, have been investigated by optical polarizing microscope, electron-probe micro-analyzer, back-scattered electron imaging, infrared spectroscopy, and single-crystal X-ray diffractometer from the standpoint of crystal growth. This grossular shows one-to-one correlation between natural surface features and its internal textures under crossed polarizers. Electron-probe micro-analyzer (EPMA) gave average chemical composition in (110) thin section, of bright lamella {Ca_2.97Mn_0.06}_∑3.03 [Al_1.59Fe_0.37Ti_0.01]_∑1.97(Si_3.00)_∑3.00 (Gros_79.5And_18.9Sps_1.6) and of dark host {Ca_2.99Mn _0.06}_∑3.05 [Al_1.73Fe_0.26 Ti_0.01]_∑2.00(Si_2.97OH_0.03)_∑3.00 (Gros_85.4And₁₃Sps_1.6). The correspondence of surface features and the internal textures with spiral or pyramidal growth mechanism suggest that the internal textures of the Eden Mills grossular are formed during growth process. The optical vibrational orientations and the growth steps inclination along [001] and ( left[overset{-}{1}10right] ) directions predict monoclinic symmetry. With X-ray diffractometer (XRD) method, pseudocubic parameters are a = 11.839(2) Å, b = 11.855(1) Å, and c = 11.868(2) Å with interaxial angles α = 90.00(1)°, β = 89.99(1)°, and γ = 90.02(2)° that show orthorhombic symmetry of this crystal. Lamellar texture of Al³⁺-rich host with Fe³⁺-rich lamella infers cation ordering at octahedral site of the garnet structure. IR data favors the non-cubic orientation of [(OH) ₄] at tetrahedral position in this grossular structure.