Metamict gadolinite–(Y) from Ratnapura, Sri Lanka

A fairly large (ca. 15 cm), massive glassy specimen of nearly black bulk colour, found in Ratnapura district, was identified as metamict gadolinite–(Y) based on its chemical composition Y_1.4Ln_0.6Ca_0.1Fe_0.9Be₂Si₂O₁₀ (with Ln = lanthanide elements La–Lu) and since the material reconstituted to monoclinic gadolinite–(Y) after two days of dry annealing in air at 1150 °C. Thin slabs (< 0.5 mm) of the specimen are bottle-green and transparent. Apart from surface weathering and small inclusions, the material is fairly homogeneous and does not show any indication of chemical alteration. Concentrations of Th (6310 ppm) and U (3360 ppm) in the material, and its calculated time-integrated alpha dose of ca. 8.5 × 10¹⁸ g^–1 based on the ²⁰⁶Pb/²³⁸U age of ca. 518 Ma, explain the present glassy state as being caused by radioactive self-irradiation over geological periods of time.