page_title: Siilinjärvi carbonatite
section_title: Micas
hierarchical_section_title: Siilinjärvi carbonatite / Minerals of the Siilinjärvi intrusion / Micas
caption_attribution_description: Scan of the thin section.English: Optical mineralogy: Photomicrographs of thin section from Siilinjärvi apatite ore. Tetraferriphlogopite grains in thin section H10. The sample is only slightly deformed (shear grade 3), and the mica flakes are bent, but still mainly unbroken. Photomicrograph from thin section in cross and plane polarised light.
context_page_description: The Siilinjärvi carbonatite complex is located in central Finland close to the city of Kuopio. It is named after the nearby village of Siilinjärvi, located approximately 5 km west of the southern extension of the complex. Siilinjärvi is the second largest carbonatite complex in Finland after the Sokli formation, and one of the oldest carbonatites on Earth at 2610±4 Ma. The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss. The maximum width of the body is 1.5 km and the surface area is 14.7 km². The complex was discovered in 1950 by the Geological Survey of Finland with help of local mineral collectors. The exploration drilling began in 1958 by Lohjan Kalkkitehdas Oy. Typpi Oy continued drilling between years 1964 and 1967, and Apatiitti Oy drilled from 1967 to 1968. After the drillings, the laboratory and pilot plant work were made. The mine was opened by Kemira Oyj in 1979 as an open pit. The operation was sold to Yara in 2007.
  The Siilinjärvi apatite mine is the biggest open pit in Finland. Currently the mine comprises two pits; the larger southern Särkijärvi and the smaller northern Saarinen satellite pit.
context_section_description: The most common mica mineral in Siilinjärvi complex is tetraferriphlogopite, which comprises 65% of the intrusion. Some glimmerites contain over 90% tetraferriphlogopite. The colour of the mineral is black or greenish black, dark brown or reddish brown. The colour is dependent on the host rock and the intensity of the deformation of the rock. The reddish brown mica usually occurs with the carbonate-glimmerites and the black mica occurs with the glimmerites. Phlogopites show very strong red-brown to pinkish yellow reverse pleocroism, which is due to high Fe³⁺ contents. The phlogopite of Siilinjärvi is sold as soil conditioner under the trade name “Yara biotite”.
  Phlogopite occurs as disseminated flakes, tabular crystals and lamellar or foliated aggregates. The grain size of the micas varies from only a couple of µm to several centimetres, the average size is 1–2 mm in diameter. The phlogopite is altered into brown biotite-phlogopite in the shear zones, and in the most intensely sheared zones, into biotite and chlorite. The most common inclusion mineral in micas is magnetite, but generally the inclusions are rare. Some zircon inclusions can also be found.
caption: Tetraferriphlogopite grains. Photomicrograph from thin section in cross and plane polarised light.
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