siltite

   A compact, weakly metamorphosed rock formed by alteration of siltstone, mudstone, or silty shale. Siltite is more indurated than mudstone or shale and lacks either shale fissility or slate-like cleavage. Siltite differs from argillite in that silt-size grains (0.002 to 0.062 mm) dominate the matrix rather than clay-size particles (<0.002 mm). Siltite differs from siltstone, mudstone, or shale in that it exhibits very low to low grade metamorphic or diagenetic layer silicate and feldspar alteration to sericite, chlorite, and albite (subgreenschist to greenschist metamorphic facies) (Maxwell, 1973; Kidder, 1987).

   SW