Foliated metamorphic rocks have visible layered or banded structure due to the alignment of minerals into parallel planes. Non-foliated metamorphic rocks lack this banded appearance and have a more random arrangement of minerals. There are several types of non-foliated metamorphic rocks including marble, quartzite, hornfels, novaculite, and slate.
Marble is formed when limestone is subjected to heat and pressure causing it to recrystallize. The calcite minerals making up limestone are transformed into larger, interlocking calcium carbonate crystals that do not have any alignment or banding. This results in marble's characteristic granular texture. Impurities in the original limestone rock will often produce colored mineral groupings in the marble.
Quartzite starts out as quartz sandstone before metamorphism causes the quartz grains to fuse together. The individual sand grains recrystallize and interlock tightly, destroying any porosity the original sandstone had. The lack of banding and massive structure of quartzite comes from this recrystallization process. Quartzite is much harder and denser than the sandstone it formed from.
Hornfels is produced when shale, mudstone or some other clay-rich sedimentary rock is heated by nearby igneous intrusions. The extreme heat bakes these rocks causing new minerals to grow in the spaces between the clay particles. This results in a fine-grained, dense, and tough metamorphic rock that retains some of the original sedimentary structures. The random crystallization gives hornfels an unbanded appearance.
Novaculite forms from the metamorphism of chert which is made of microcrystalline quartz. Increased heat and pressure causes the tiny quartz crystals to grow larger and tightly interlock. This erases any sedimentary banding, forming a hard, dense, white to gray rock. Novaculite works well as a sharpening stone due to its fine but abrasive texture.
Slate originates from shale metamorphosed by relatively low levels of heat and pressure. The planar clay mineral layers in shale are realigned perpendicular to the pressure during metamorphism. This realignment combined with microscopic recrystallization of minerals gives slate its trademark splintery cleavage. Despite this cleavage, slate still lacks any visible banding making it non-foliated.
The common factor in all these rocks is that increased heat and pressure has caused the minerals to recrystallize and grow together, obscuring any original sedimentary textures, bedding planes, or flow banding. Without any distinct foliation, these metamorphic rocks appear massive and structureless apart from their mineral composition and grain size. This sets them apart from foliated metamorphic rocks like gneiss, schist, and phyllite that show visible layered or banded characteristics. The exact temperatures, pressures, and original rock compositions will dictate which type of non-foliated metamorphic rock ultimately forms.
