Nonfoliated metamorphic rocks are formed around igneous intrusions where the temperatures are high but the pressures are relatively low and equal in all directions (confining pressure). The original minerals within the rock recrystallize into larger sizes and the atoms become more tightly packed together, increasing the density of the rock.
Nonfoliated metamorphic rocks are identified on the basis of their composition:
Quartzite = composed of the mineral quartz; metamorphosed sandstone
Marble = composed of the mineral calcite; metamorphosed limestone