Chapter 2 Scottish mineral Geological Conservation Review Sites – Magmatic Minerals

Abstract

Following the pioneering work of James Hutton in the 18th century, which was largely carried out on Scottish rocks and minerals, it became generally accepted that igneous rocks formed from liquid magmas. The selected 10 sites cover a wide range of magmatic events, rock types and mineral assemblages. In the case of basic and ultrabasic rocks, prolonged cooling and repeated injections of fresh magma into the magma chamber can lead to settling of early crystals resulting in a rhythmic mineral layering and accumulations of mineral species, such as those bearing chromium and nickel, which have economic importance. The former chromite quarries at Hagdale and Nikka Vord, on Unst ably illustrate the size and podiform nature of the massive chromite bodies and also contain examples of chromium secondaries, such as chromian clinochlore, whereas the water-worn rocks on the shore at Wick of Hagdale provide clean exposures of banded and disseminated chromite. Podiform chromite also features in the Corrycharmaig serpentinite, but the focus of interest there is on the extensive replacement of the original mineralogy by talc and ferroan magnesite. A somewhat different magmatic mineral assemblage is present at Talnotry where contamination of a noritic magma by sedimentary rocks led to the precipitation of Cu-Ni sulphides and platinum group minerals.Prolonged fractionation of mafic minerals in basic magmas can lead to the production of anorthosite. Since such rocks are extremely rare in Scotland the inclusion of the South Harris Anorthosite (which also includes gabbro) in the Scottish Mineral sites is particularly apposite. Mineral chemistries in igneous rocks can provide important indications of cooling histories and formational conditions, particularly with regard to depth of origin. Thus the presence of large zoned clinopyroxene crystals in the volcanic rocks at Bail Hill and sanidine phenocrysts, of appreciable size in a trachybasalt dyke at the Bangley Quarry, both within extremely fine-grained matrices, demonstrate development in a differentiating magma at depth before emplacement. The remaining two sites included in this chapter contain material which provides a window into the upper mantle below Scotland. These comprise pyrope and zircon xenocrysts along with ultramafic xenoliths in the volcanic ventat the Elie Ness site and large corundum (sapphire), sanidine, phlogopite, biotite and magnetite megacrysts in the Cnoc a’ Chapuill-Loch Roag dyke.