Prehnite occurs in amygdules and veins in low-grade meta-basic rocks (e.g., basalts) but is more common as a constituent of low-grade meta-greywackes and meta-basic rocks where it is typically too fine-grained for identification by optical techniques.  It is colourless in plane light (A) and has moderately high positive relief relative to quartz and sodic plagioclase, with which it is commonly associated.  In image A prehnite dominates the amygdule and stands out relative to minor interstitial quartz and a quartz core.  In amygdules and veins it commonly forms radiating sheaves, apparent in crossed nicol views B and D.  Maximum interference colours range to upper second order and extinction is parallel, or nearly so, to the length of the plates in the bundles.  All views are 5.5 mm across.  A, C ppl, B, D x-nicols.