It forms by precipitation from ground and surface waters, forming the major component of marls and limestone in the ocean, lakes and rivers. It also forms speleothemes in caves, euhedral crystals in vugs, and tufa where springs emerge on the surface. It is the major component of marbles, which forms by the metamorphism of limestones and other carboate-rich rocks. Calcite also is commonly found in hydrothermal veins. It may occur as a rare accessory in syenitic or pegmetitic igneous rocks.
Space allows only a few of the most remarkable calcite occurrences to be mentioned here.
Most of the state's calcite occurrences are in a region underlain by Paleozoic carbonate rocks, which extend in a wide curving band across the state from St. Croix County in the west, south along the Mississippi Valley east to Madison, then north to Green Bay and the Dore Peninsula. Granular, sparry calcite is a major constituent of the Bad River marble found throughout the Penokee Range. Here it is associated with dolomite, chlorite, quartz, sericite, tremolite and pyrite. Some outcrops include SE NW sec. Granular, sparry calcite is a major constituent of the Bad River marble found throughout the Penokee Range.