Return to Alkaline Rocks
In alkaline rocks, amphibole-group minerals (or amphiboles, for short) come in a variety of colors and crystal morphologies (which, for obvious reasons, we cannot review exhaustively here). In agpaitic rocks, the most common types are sodic and sodic-calcic amphiboles with various proportions of K, Mg, Fe2+, Fe3+, Ti, Al and Mn. They are usually black in hand-specimen, but reveal a kaleidoscope of colors under the microscope. The colors replace one another as the microscope stage is rotated; this orientation-dependent color variation is called pleochroism. The most typical pleochroic palette of agpaitic amphiboles ranges from dark blue (often nearly black), teal or bluish green to greenish brown, sienna or "peachy" orange (see at right). High amounts of Ti "dye" amphiboles in various shades of tangerine-orange. Shown here are the sodic amphibole magnesio-arfvedsonite and aegirine in lujavrite from the Lovozero Mts. Both minerals are black and prismatic in hand specimen, but can never be confused under the microscope. The grass-green to lemon-yellow pleochroism of aegirine and its coarse 90o cleavage are never seen in amphiboles.