Scapolite is a common rock-forming mineral in parts of the Belt Series of Precambrian age in southern Shoshone County and adjoining parts of Clearwater County, Idaho. It is most abundant in moderately metamorphosed calcareous shaly layers of the Wallace Formation but occurs also in their highly metamorphosed equivalents and in the lowest part of the Prichard Formation.

The mode of occurrence varies with distance from the Idaho batholith, with bulk composition, and with grade of metamorphism. In the northern part of the area where rocks were metamorphosed to the epidote-amphibolite facies, the highest concentration of scapolite is in layers rich in calcite, biotite, or hornblende and diopside. In biotite-rich layers interbedded with quartzite, scapolite is in round holoblasts; in carbonate granofels, crystals are euhedral to subhedral; and in hornblende- and diopside-bearing layers, small anhedral grains are common. In rocks metamorphosed to the amphibolite facies, such as diopside gneiss and calcium-magnesium-aluminum silicate rocks (here named “camalsite”), scapolite occurs in small anhedral grains. In diopside gneiss, scapolite is in thin layers that extend long distances parallel to the bedding. In camalsite, small masses exceptionally rich in scapolite are common.

The mode of occurrence and the distribution parallel to the bedding suggest that scapolite crystallized from a sedimentary rock that contained saline minerals. The scapolite contains much Cl but only a little SO₃, suggesting that halite was the chief source mineral. The local high concentrations of scapolite occur in a zone where elements have been redistributed, either because of metamorphism of dispersed or layered saline minerals with accompanying migration of chlorine or because of metamorphism of primary local masses of such minerals.