The geognosy of Scottish tourmalines

Abstract

An extensive series of specimens containing Tourmaline forms part of the Scottish Mineral Collection which has been under my charge in the Edinburgh Museum of Science and Art during the past thirteen years. As these specimens present some features of more than a purely mineralogical interest some notes upon them may well be placed before this Society. Tourmaline is essentially a Rhombohedral Boron-Aluminium Subsilicate, which contains either Magnesium or Iron, or else an Alkali. It may be regarded as a name for a small group of subsilicates of somewhat diverse composition, in which Boron, in the proportion, in some cases, of nearly thirteen per cent., is combined with Aluminium, and with other bases. Thus there are recognised Alkali Tourmalines, Iron Tourmalines, Magnesium Tourmalines, and even Chromium Tourmalines. Fluorine and Lithium occur too in so many cases that they may almost be regarded in the light of essential constituents. There are about thirty native Boron compounds known, of which twenty-five or so are borates of the metals. Only two of the thirty have hitherto been recorded from Scotland: the other besides Tourmaline being Datolite. As its composition may be said to be various, so is its colour, which may range through greater part of the scale, or, on the other hand, the colour may be absent altogether. The Tourmalines which contain much iron are usually dark in colour, or even black, and in this latter case they are opaque. The most usual and characteristic habit of Tourmaline crystals is that of