File:Thin section scan crossed polarizers Siilinjärvi H10.jpg

English: Scanned image of thin section from Siilinjärvi apatite ore in cross polarised transmitted light. The sample was collected from the Särkijärvi area, from the shear zone in the central-northern end of the pit.

The thin section has a hypidiomorphic structure. Sheared zones comprise 20 area-% of the thin section. The rock is only slightly oriented.

Tetraferriphlogopite forms the weak orientation in the thin section, and about 75 area-% of the thin section. Tetraferriphlogopite flakes have turned into the shear direction and broken into elongated grains towards the shear direction. The biggest grains are on average 2.5 mm long and 1.5 mm wide. The large mica flakes are bent.

Amphiboles cover 12 % of the thin section and occur as sub- to anhedral monoclinic grains. The grains are broken and have a lot of inclusions: big inclusions (about 1 mm in width) of apatite and mica, and smaller (less than 0.1 mm in width) inclusions of carbonates. Occasionally, the amphibole grains are altered from the sides. The biggest poikiloblastic grains are > 10 mm wide, but the grain size varies a lot. The blue colour of the mineral grains is quite weak, but still prominent – the amphiboles are a mixture between tremolite and richterite.

Apatite is present in quite small rounded porphyroclasts (usually less than 1 mm wide), the bigger grains are often associated with carbonates. Apatite forms about 10 area-% of the thin section. Large grains have inclusions of carbonates and tetraferriphlogopite.

Carbonates are mostly found as inclusions in apatites and amphiboles. Both carbonate minerals are equally common. They form 3 area-% of the thin section. The carbonate inclusions are elongated in amphibole grains, but in apatite grains the boundaries between carbonates are hard to distinguish.