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Minerals of base metals
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Ilmenite is the most important commercial source of titanium [685]. It is a titanium-iron oxide mineral which is a weakly magnetic black or steel-grey solid with the idealised formula FeTiO3. It contains appreciable quantities of magnesium and manganese with the full chemical formula as (Fe,Mg,Mn,Ti)O3 and forms a solid solution with geikielite (MgTiO3) and pyrophanite (MnTiO3) which are the respective magnesian and manganiferrous end-members of the solid solution series. Leucoxene, an important source of titanium in heavy minerals sands ore deposits, is an altered form of ilmenite. It is a typical component of altered gabbro and diorite that is generally indicative of ilmenite in the unaltered rock.
Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia
Published in Australian Journal of Earth Sciences, 2018
R. C. Dwyer, W. J. Collins, A. C. Hack, R. Hegarty, H.-Q. Huang
The calc-alkaline group is characterised by several apparently comagmatic dolerite and gabbro intrusives from MINOL21 and MINOL33 that are noticeably different from the alkaline group (Figure 3c, d). Granular equant to tabular pyroxene appear as 3–8 mm phenocrysts, but range to groundmass size, whereas plagioclase appears as an interlocking network of equant to tabular laths. Opaque minerals (magnetite/ilmenite) are commonly altered to leucoxene. Clinopyroxene crystals show only faint pale brown pleochroism diagnostic of augite, and are thus distinct from the pink-purple pleochroism of titanaugite in the alkaline rock group (Figure 3a, b). Deformation and alteration are more intense in calc-alkaline, compared with alkaline, rocks. Localised shearing occurs in a clinopyroxene-phyric ankaramite at the base of MINOL21 and provides a reasonably equivalent sample to compare with the pristine ankaramite from PANGBH5. Mylonitic S–C fabrics are developed in the ankaramite from MINOL21, whereas phenocrytic augite and the groundmass within this unit are highly altered. Elsewhere in associated tuffs, sedimentary rocks and field samples, bedding parallel and axial planar fabrics were observed (MINOL21-05, MINOL30-04, MINOL33-06, RDT7). Unlike the alkaline rocks, alteration assemblages partially to completely replace the primary igneous mineral assemblages and in some cases obscure the original calc-alkaline igneous texture. See Supplementary Papers (Table SM2) for petrographic summaries of key samples.