File:Thin section scan crossed polarizers Siilinjärvi R625-145.40.jpg

English: Scanned image of thin section from Siilinjärvi apatite ore in cross polarised transmitted light.

The thin section is highly sheared and quite homogeneous. There is an open fracture in the middle of the thin section. The grains are small and quite equally distributed.

Phlogopite forms half of the thin section area and it is present as small elongated laths. According to pleochroism, the phlogopite is not common phlogopite nor tetraferriphlogopite, the phlogopite composition is between those minerals. The laths are oriented, and large flakes are rare. The orientation is more uniform in the shear planes. The grains are almost always < 1 mm long and 0.2 mm wide, often 0.1-0.5 mm long and < 0.1 mm wide.

Carbonates are present all over the thin section, but among some regions they form vein like granular aggregates. Granular aggregates consist mainly of calcite, and individual fragments are commonly dolomites. Calcite is more common carbonate mineral than dolomite. The carbonate grains are partially altered. Calcite grains in the aggregates are 0.2-1.5 mm in diameter, i.e. often larger than the individual dolomite fragments in the mica flow. Some individual grains are up to 1 mm wide, but usually the fragments are 0.1-0.5 mm in diameter, or even smaller. Carbonates form 35 area-% of the thin section.

Apatite is mostly present as fragments in the mica flow and forms 7 % of the thin section area. The grains are 0.1-1 mm in diameter.

Amphiboles form also 7 % of the thin section area and are present in the flow zones as chains of broken particles. The grains are small, < 0.2 mm wide, and occur as elongated bars or euhedral lozenges. Elongated grains have a preferred oriented and can be up to 1 mm long.