Zeolites are common as low-temperature hydrous alteration products in volcanic tuffs, other low-grade metamorphic rocks and as cavity fillings in volcanic rocks. Chabazite is shown here as an example of the latter occurrence.  It forms small colourless cube-like rhombohedra lining the wall of a vesicle in this example (A).  Refractive indices are low (n = 1.46 - 1.52), and the chabazite has moderate negative relief relative to the epoxy.  Chabazite has extremely low birefringence, seen here as faint greys to black that are almost indiscernible relative to the strained and dusty epoxy that fills the remainder of the vesicle in B.  Sample is of a vesicular phonolite from Mt. Kenya.  Views are 2.2 mm across.  A ppl, B x-nicols.