File:Thin section scan crossed polarizers Siilinjärvi R625-47.40.jpg

English: Scanned image of thin section from Siilinjärvi apatite ore. Crossed polarizers.

The rock is oriented and porphyroclastic. The micas and amphiboles are clearly oriented, and the carbonates have grown into vein-like structures.

Tetraferriphlogopite is present as small laths and porphyroclasts of large flakes. Laths are commonly 0.2-0.5 mm long and less than 0.1 mm wide. Bigger grains are up to 3 mm in diameter, and exist unto lath-sized particles. Oriented laths form roughly 60 % of the total area of tetraferriphlogopites, and in total the mineral forms 70 % of the thin section area.

Carbonates cover 15 % of the thin section. Dolomite is more common carbonate mineral than calcite. Carbonates are found as individual grains, as granular aggregates, and in companion with apatite. Calcite occurs commonly as big aggregates, and dolomite is commonly found as individual grains. Usually the carbonate grains are quite small, but some grains in the middle of the aggregates are up to 1.5 mm in width. Average sized grain is < 0.5 mm in width. Carbonates show almost no signs of alteration.

Apatite forms 10 area-% of the thin section. They are found as rounded grains, broken grains, and fragments. Bigger grains have some inclusion trails of carbonates. Grains are found in all sizes between tiny fragments and large porphyroclasts (> 0.1-3 mm).

Richterite amphiboles are widely oriented and are found as elongated bars and euhedral lozenges. Grain size varies a lot: the biggest grains are > 3 mm in diameter meanwhile many broken fragments are < 0.1 mm wide. The richterite grains cover 5 % of the thin section.