Keele Research Repository

The Gairloch area lies at the southern end of the Lewisian outcrops in the N.W. of Scotland. The rocks of the Gairloch area are metasediments and belong to the Loch Maree Group. In these rocks lenses of Iron Formation facies occur. This Iron Formation is divided into two facies, the oxide facies (consisting of magnetite and quartz) and the silicate facies (consisting of garnet and amphibole). These minerals are coarsely crystalline and form a banded texture in the oxide facies, and a granoblastic or porphyroblastic texture in the silicate facies. In the associated metasediments, the textures are either lepidoblastic (biotite-muscovite schist) or nematoblastic (hornblende schist).
Grunerites, garnets, magnetites and rocks from the Gairloch Iron Formation and the associated metasediments have been analysed using X-ray fluorescence, X-ray diffraction, the electron microprobe and wet chemical methods for both major and trace elements. The cell parameters have been determined for the minerals. The analyses show that the amphiboles are grunerite and manganoan grunerite, and the garnets are either almandine or spessartite-almandine. These coexisting minerals are interpreted as being in equilibrium and the Fe 2+ and Mg2+ are preferentially incorporated into the grunerite structure whilst the Mn 2+ and Ca2+ are partitioned into the garnet structure. The origin of these silicate minerals in the Gairloch Iron Formation is assumed to be from the original hydrous iron silicates and clay minerals. The trace elements Cr, V and Y are concentrated in the garnet. Nb, Ni, Sc, Sr and Zr have approximately equal proportions in the garnet and grunerite. Ba, Pb and Rb do not occur in these minerals but they occur in minerals from the associated metasediments. The magnetite from the oxide facies of the Gairloch Iron Formation seems to be free of trace elements.