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Low-temperature samarskite modification: structural relationship between pre-metamict and recrystallised structure

Previous investigations on samarskite proposed two structural modifications: low-temperature samarskite, being observed at temperatures up to 550 °C, and high-temperature samarskite, which occurs at temperatures above 950 °C [1]. A sample of samarskite-(Y) from Ampangabe, Madagascar, was investigated to trace relics of the original structure in metamict matrix and to relate them to the sample recrystallised in air at temperatures around 500 °C. Investigations performed by high-resolution electron microscopy (HRTEM) revealed occurrence of regions containing relics of pre-metamict structure. These relics were surrounded by amorphised mineral matrix (Fig. 1). Selected area electron diffraction (SAED) of these regions reveal an orthorhombic unit cell having following unit cell parameters: a = 5.66 (4) Å, b = 4.97 (2) Å, c = 5.28 (6) Å, V = 149 (2) Å3. X-ray diffraction (XRD) pattern of the thermally untreated sample is typical for heavily metamictised minerals. Four low-intensity diffraction maxima were observed that could be attributed to the low-temperature samarskite modification. X-ray diffraction pattern of the sample annealed at 500 °C revealed mostly recrystallised low-temperature samarskite with an orthorhombic unit cell: a = 5.58 (2) Å, b = 4.92 (2) Å, c = 5.19 (2) Å, V = 142.3 (7) Å3. A comparison of HRTEM and XRD data indicates a structural relation of the preserved pre-metamict structure and the recrystallised mineral at 500 °C. This can be used as a prerequisite in a structure solution for samarskite samples. [1] Sugitani Y. et al. (1985) Am. Mineral. 70, 856- 866.

samarskite; low-temperature modification; TEM; XRD

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