The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms. Schlieren are usually interpreted as having arisen by one of four mechanisms:
1. shearing of heterogeneities (enclaves or xenoliths),
2. crystal sorting during convective flow,
3. crystal sorting during magmatic flow, or
4. crystal settling.
At the time of formation or crystallization of a magma chamber, mafic minerals such as biotite, rare earth elements of the lanthanide and actanide series, allanite, and the phosphate mineral apatite can orient in a preferred manner that creates bands. Schlieren bands vary in geometry ranging from deformed, tubular, planar, and rings, to arachnid (spider-like) formations.
A schlieren arch is an intrusive igneous body with flow layers that occur along its borders, but which are poorly developed or absent in its interior. A schlieren dome is an intrusive body that is almost completely outlined by flow layers that culminate in one central area.
[images: schlieren in biotite-rich mantle with granodiorite inside and outside, and close-up of the margin of the schlieren; a prominent schlieren that defines a structure rather like the hinge region of an isoclinal fold, and close-up of the upper left side of the schlieren showing dark, biotite-rich prominent part of the schlieren (curving to the right) with thinner, less prominent biotite -rich streaks extending upwards (the K-feldspar phenocrysts are approximately parallel to the schlieren margin); spidery "arocknid", composed of two sprays of thin schlieren; thick portion of schlieren with irregular convex surface, parallel alignment of K-feldspar phenocrysts, and K-feldspar phenocrysts in the host granodiorite that are nearly perpendicular to the convex margin of the schlieren (top center); K-feldspar-rich mass in normal foliated granodiorite; schlieren.
Schlieren (from the German for 'streaks') are optical inhomogeneities in transparent material that are not visible to the human eye. Schlieren, shadowgraph, and interferometric techniques are used to study the distribution of density gradients within a transparent medium.