This is a rock containing a variety of minerals related to its original premetamorphosed composition as a volcanic tuff. Halleflinta, therefore, contains quartz and has been enriched with silica during metamorphism.
It is frequently pale-colored and can vary from brown to pink, green, gray, or yellowish brown.

TEXTURE Halleflinta is a fine-grained rock— a microscope is needed to study its mineral composition. Texture is even, with a flinty, crystalline appearance. This rock breaks with a sharp, splintery fracture. It may show a layered structure related to the original stratification of the volcanic tuff. Porphyroblastic textures with large, isolated crystals are sometimes found.

ORIGIN Forms by the contact metamorphism of tuffs, which have usually been impregnated by secondary silica. It is often associated with hornfels.

Group: METAMORPHIC

Grain size: Fine

Classification: Contact

Pressure: Low

Temperature: High

Structure: Crystalline