File:Thin section scan crossed polarizers Siilinjärvi R636-42.60.jpg

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

The thin section is highly sheared and has a fine grain size. Mineral crystals have broken into small fragments, and almost only remaining structures are carbonate aggregates. Tetraferriphlogopite covers half of the thin section area and is mostly present as tiny elongated and oriented laths. The laths are usually 0.1-0.5 mm long but some bigger flakes are remaining with diameters up to 2 mm.

Carbonates are present as granular mineral aggregates but also as fine-grained mash with apatite. Calcite is the dominating carbonate mineral. The grain diameter in the aggregates is 0.2-1 mm and the grains are quite well-preserved. Outside the aggregates the grains are much smaller and in companion with apatite; these grains are commonly dolomites. Carbonates form 30 area-% of the thin section.

Apatite is mostly found as scattered small fragments. Apatite covers 15 area-% of the thin section, and is often in companion with carbonates. Fragments are mostly < 0.5 mm in diameter, but some still remaining larger grains can be up to 2 mm in diameter. Usually the fragments are 0.05-0.2 mm in diameter.

Richterite amphiboles and opaques are featured as accessory minerals. Richterite grains are often seen as small elongated slivers or euhedral lozenges in the mica flow areas. Euhedral crystals are < 0.3 mm in diameter, elongated grains can be slightly larger. Opaques are probably mostly magnetite.