The term serpentine refers to three polymorphs (lizardite, antigorite, crysotile) that may occur together and cannot be easily distinguished in thin section.  Serpentine primarily forms by hydrous alteration of olivine, and in some cases pyroxene, and many crustal peridotites are extensively altered to serpentine.  It is typically colourless to pale yellow or pale green in plane light, and is commonly intergrown with opaque magnetite (A).  The very low maximum interference colours of first order white to yellow are evident in B.  In C, the original outlines and internal fracture pattern of olivine grains are highlighted by opaque magnetite.  (Coarse subhedral opaque minerals are primary Cr-spinels grains.)  In D, more highly birefringent calcite and tremolite accompany the serpentine. Serpentine can be confused with pale-coloured varieties of chlorite (Mg-rich), but in serpentine the slow ray is parallel to the fiber length whereas chlorites with Mg/(Mg+Fe) < 0.50 are length fast.  A and B are from the Hunting Hill, Maryland serpentinite and C and D are from a serpentinized peridotite cumulate from the Abitibi Belt, Ontario.  A and B are 2.2 mm across and C and D are 5.5 mm across.  A and C ppl, B and D x-nicols.