THE ROLE OF TERNARY PROJECTIONS IN COLOR DISPLAYS FOR GEOCHEMICAL MAPS AND IN ECONOMIC MINERALOGY AND PETROLOGY

Ternary (triangular) diagrams show the compositional variation of thre e end-members, recalculated to 100%, and represent the projection into two dimensions of three or more components (since an end-member may r epresent the sum of more than one original component), and hence multi ple space. During the present century they have been of particular imp ortance in chemistry, the earth sciences, colour-mixing and the studie s of colour that underlie the development of geochemical colour maps. Such concepts can be traced back to Newton's Opticks although he did n ot use an explicit ternary theory of colour mixing. Development of the trichromatic theory of colour deficiency began with Mariotte in the 1 7th century and was definitively established by Maxwell in the 1850s. These colour theorems underlie the development in recent years of the colour maps now in widespread use in geochemistry. The progress of reg ional geochemical mapping in the British Isles has gone hand-in-hand w ith the development of computer software and hardware. The use of mult i-element colour-combined geochemical maps can be traced from early wo rk at Imperial College by Webb and his co-workers (1964-79) to its lat er application in the British Geological Survey's Geochemical Atlas se ries (from 1983 onwards). Between 1955 and 1975 the use of ternary dia grams came to be commonly used in studies of mineralogical composition , including economic minerals, petrology, classification, and phase re lationships. In igneous petrology, the quartz-alkali-feldspar-plagiocl ase-feldspathoid double-triangle has achieved increasing international acceptance. We illustrate these applications with examples pertinent to the Northern Highland Caledonides, with particular emphasis on the lamprophyres and associated rocks. Two distinct multivariate trends of variation are shown to be present which may warrant further investiga tion to elucidate their bearing on Caledonian mineralisation. (C) 1998 Elsevier Science B.V. All rights reserved.