Turbid alkali feldspars from the Isle of Skye, northwest Scotland

Abstract

Partially turbid alkali feldspars from hydrothermally altered Tertiary granites on the Isle of Skye (the Red Hills granites) were studied using light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. Limpid cores and turbid rims of individual crystals were compared to determine the causes of the turbidity. The limpid cores were cryptoperthitic, with lamellar widths of 0.1–0.3 μm. In contrast, the turbid rims contained K-rich and Narich areas coarsened to >0.5 μm. Turbid regions contained abundant inclusions, whereas limpid regions did not. Two generations of turbidity were recognized. Feldspars from the Beinn an Dubhaich granite, a granite with near-normal values for ¹⁸O/¹⁶O possessed limpid cores surrounded by turbid rims that cast a reddish-brown hue in transmitted light. When viewed in darkfield light microscopy, the regions with the reddish-brown turbidity were blue. This is consistent with the hypothesis that the cloudy appearance of these turbid regions arises from the scattering of light by micrometerto submicrometer-sized inhomogeneities in refractive index caused by fluid-filled cavities. Feldspars from the Loch Ainort granite, a granite with low values for ¹⁸O/¹⁶O possessed limpid and reddish-brown-turbid cores surrounded by turbid rims that cast a blackish hue in transmitted light. Ion thinning of the turbid areas produced an abundance of small holes (≤1–2 μm) apparently the remains of fluid inclusions. Transmission electron microscopy revealed that some holes from regions of reddish-brown turbidity contained non-feldspar material, including halite and metal-rich phases of various compositions. In contrast, blackish turbid regions contained cavities filled with alteration products, such as kaolinite. Hence, the feldspars from granites on the Isle of Skye apparently record interactions with at least two fluids: a saline fluid (possibly a late-stage magmatic fluid) and a meteoric fluid.