The chemistry and mineralogy of the olivine nodules of Calton Hill, Derbyshire

Abstract

Summary Chemical analyses, optical data, and cell parameters are given for olivine (Fo 90·3 and Fo 91·4 ), orthopyroxene (Ca 1·5 Mg 89·0 Fe 9·5 and Ca 9·8 Mg 88·6 Fe 10·6 ), clinopyroxene (Ca 39·6 Mg 55·8 Fe 4·6 ), and chromian spinel from olivine nodules from the Carboniferous basalt of Calton Hill, Derbyshire. Chemical and modal analyses of the nodules and of their host rock are also presented. The distribution of Mg and Fe in the coexisting pyroxenes is fairly similar to that observed by Ross, Foster, and Myers (1954) but differs slightly in that the extension of the pyroxene tie-line intersects the Ca-Mg side of the Ca-Mg-Fe triangle. The clinopyroxene is a normal augite rather than the hydrous augite previously reported (Tomkeieff, 1928). The high chromium content of the spinel and clinopyroxene contrasts with the low value for this element in the host rock, suggesting that these minerals are not differentiates of the basaltic magma: the nodules are considered to represent fragments from a deep-seated peridotite.