COLOUR INDEX

in astronomy, the difference between two measurements of the magnitude (brightness on a logarithmic scale) of a star made at different wavelengths, the value found at the longer wavelength being subtracted from that found at the shorter. Usually the two wavelengths are the blue, measured from ordinary photographic plates that are most sensitive to blue light, and the visual, measured on photographic plates sensitized to yellow. The index is a measure of a star's colour, an indication of its temperature, and a fairly crude description of the distribution of its radiated energy through the electromagnetic spectrum. Hot, blue stars have negative colour indices, as they radiate most strongly and therefore have numerically lower magnitudes at short wavelengths, and those of cool, red stars are positive. The colour index of a star is increased by the passage of its light through interstellar matter; the amount by which it exceeds the normal value for its spectral type is called the colour excess. See also magnitude; UBV system. in igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals contained by the rock. Volume percentages, accurate to within 1 percent, can be estimated under the microscope by using a point-counting technique over a plane section of the rock; volumes also can be approximated visually in hand specimens in the field. As originally presented, the terms felsic and mafic were used in a broadly descriptive sense to indicate the relative abundances of light-coloured and dark-coloured minerals, respectively, in an igneous rock. The most common light-coloured minerals are the feldspars, feldspathoids, and silica or quartz, giving the term felsic; other felsic minerals are corundum, zircon, muscovite, lepidolite, and calcite. The abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals. Most minerals fall within these two broad groups. Broadly speaking, mineral colour indicates the specific gravity of the mineral; minerals that are lighter in colour are also lighter in weight. Darker minerals typically contain more of the relatively heavy elements, notably iron, magnesium, and calcium.