colour index


colour index
In igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals in the rock.

The most common light-coloured minerals are feldspars, feldspathoids, and silica or quartz; abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals.

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      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 (B) and the visual (V) as defined in the UBV system.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. The zero point of the colour index scale in the UBV system is chosen such that stars that have a surface temperature of 7,400 K and that are white in colour, such as Vega, have a colour index of zero. 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 and 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 (mafic rock) 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 (felsic and mafic rocks); 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.

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Universalium. 2010.

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