Olivine alters readily in the presence of water to serpentine, and olivines in most crustal rocks are serpentinized to at least some degree.  In the example illustrated here, subhedral to anhedral olivines are extensively replaced by serpentine, with the olivine occurring as relict high-relief colourless grains surrounded by pale-green, low relief regions of serpentine (A). The olivines are enclosed by altered intercumulus clinopyroxene. The opaque mineral is magnetite that formed during serpentinization.  Viewed under crossed nicols (B), all portions of remnant olivine that were part of a single grain before serpentinization remain in optical continuity though they are now separated by serpentine.  This is best illustrated in the large grain in the upper centre of these images, in which the fresh olivine has second order interference colours.  The two olivines in the lower portion of these images only show first order grey interference colours (due to their orientation), which are difficult to distinguish from the serpentine that is replacing them.  Sample is from the Muskox Intrusion, N.W.T.  Images are 5.5 mm across.  A ppl, B x-nicols.