A Visual Basic program (TOURCOMP) has been written to recast the tourmaline composition into end-member components from electron microprobe data or more complete tourmaline analyses. TOURCOMP is a program based on a linear algebraic model that directly calculates the end-member proportions of tourmalines from their structural formulae. The program is developed for IBM-compatible personal computers running under the Windows™ operating system. The source code has been also translated and compiled in order to run on an Apple computer. Analytical problems, uncertainties concerning site occupancies, and the normalization procedure to determine the structural formula are the main error sources. However, the method of recalculating tourmaline end-members presented in this paper is considered to provide reasonably good results, bearing in mind the chemical complexity of tourmaline.

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