File:Thin section scan crossed polarizers Siilinjärvi H16.jpg

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

The thin section has lepidoblastic texture and the big grains form about 75 area-% of the thin section. Sheared zones comprise the rest 25 area-% of the thin section.

Phlogopite forms the orientation in the thin section, and about 60 area-% of the thin section. According to pleochroism, the phlogopite is not common phlogopite nor tetraferriphlogopite, the composition is between those minerals. The phlogopite flakes have turned into the shear direction and broken into elongated grains towards the shear direction. The biggest grains are on average 4 mm long and 1.5 mm wide. C’-type shear band cleavage is visible and bigger mica grains are bent.

Apatite forms about 30 area-% of the thin section and occurs as big, rounded grains. The biggest grains are on average 5 mm wide, but they are broken into smaller pieces along the shear direction. The small pieces are commonly grouped. Apatite grains, especially the bigger ones, are full of carbonate inclusions. The apatite fractures are filled with carbonates. The carbonate-filled veins are often oriented to opposite direction to mica shear direction. The broken individual pieces of apatite are in companion with mica in the shear zones, but mostly the apatite forms big zones composed mainly only of apatite and carbonates.

Carbonates are often found in company with apatite and form about 10 area-% of the thin section. Calcite is more common carbonate mineral than dolomite. Anhedral granoblastic grains are often about 0.3-0.5 mm wide and grouped with other carbonate grains. Much of the calcite is found in the cracks within the apatites. The calcite in the apatite cracks has a veined shape. Individual grains of subhedral dolomite are found outside apatite-carbonate areas.