Pyrophanite and manganoilmenite in granites of the third phase of the intrusion of the Turochak granosyenite-granite-leucogranite complex (Bolshoy Ikonostas Mountain, Gorny Altai)

The simultaneous presence of two minerals of the ilmenite group has been established in the granites of the third phase of the Turochak complex intrusion (γD1t). They differ in size, location in the rock and chemical composition. Magnetite grains occur together with them. Manganoilmenite (MnO 17.95—22.68 wt.%) occurs as euhedral grains 150—200 µm in size, confined to biotite laths. The iron-manganese ratio varies from 1.18 to 1.63. The pyrophanite minal makes up 38—45%, hematite minal is rarely present (up to 2%). The general formula is as follows: (Fe2+0.53—0.60Mn0.38—0.45Fe3+0—0.02)1.00—1.01Ti0.99—1.00O3. Pyrophanite (MnO 24.35—27.18 wt.%) is represented by small (up to 50 µm) euhedral grains located in the intergranular space. The iron-manganese ratio is less than unity (FeO/MnO from 0.74 to 0.93). The pyrophanite end-member makes up from 52 to 58%, the hematite end-member is absent. The generalized formula is (Mn0.52—0.58Fe2+0.42—0.48)1.00Ti1.00O3. Magnetite in the rock occurs as isometric euhedral grains, mainly confined to biotite, no more than 40 µm in size. Single grains are found in plagioclase; their size reaches 150—180 µm. Magnetite is characterized by the constant presence of titanium (TiO2 5.87—7.30 wt.%) and manganese (up to 0.20%), the amount of total iron is FeO 92.59—94.02 wt.%. The general formula of magnetite is Fe2+1.16—1.20(Fe3+1.60—1.68Ti0.16—0.20)1.80—1.84O4. The oxygen activity calculated for the "magnetite-manganoilmenite pair is from –26 to –24, and for the magnetite-pyrophanite pair –36 to –32, which indicates reducing conditions during their crystallization.