Barite is a common accessory mineral in hydrothermal veins associated with sulphide ore deposits, and as a precipitate in sedimentary rocks such as limestones.  It usually occurs as tabular crystals, forming radial aggregates in some cases, that are colourless in thin section (A).  The moderately high positive relief of barite is evident relative to three fluorite crystals and the epoxy-filled void in A.  The “dusty” appearance of barite in this example is due to an abundance of tiny fluid inclusions.  Barite has four cleavage directions, including perfect {001} cleavage.  The maximum interference colour is first order yellow, and barite goes extinct parallel to {001} (the long dimensions in these images).  Sample is from a fluorite - barite vein from the Noyes fluorite mine near Madoc, Ontario.  Both images are 2.2 mm across.  A ppl, B x-nicols.