Corundum-fuchsite

The high-grade terrain of southern India is known for the occurrence of corundum in a variety of modes, which include high-alumina schists and gneisses, metatroctolitic and anorthositic rocks, and metasomatic reaction zones developed between ultramafic rocks and pegmatite or felsic gneiss. Since about 2004, polished ruby corundum-fuchsite rocks with a blue and white corona texture are popular items available for purchase at the mineral shows around the world. The rocks come from a small occurrence (Fig.1) in the densely forested and deeply weathered westernmost part of the Archaean Dharwar Craton (Kodagu District, Karnataka, India), where they are illegally mined in small pits.

Fig.1: Geological map of the Mercara region. Black box = area where Corundum-fuchsite rocks are mined from small pits. From Geol. Survey of India.

In hand specimen corundum-fuchsite rock exhibits a conspicuous porphyroblastic texture defined by tabular pink corundum crystals up to 1 cm in diameter, set in a pale green fine-grained fuchsite matrix. Bluish-green kyanite commonly rims the euhedral ruby accentuating its outline. In some cases a fine aggregate of fuchsite replaces the kyanite. The kyanite-fuchsite rim is separated from the green fuchsite matrix by a round to oval shaped corona of quartz and coarser-grained muscovite. Accessory rutile occurs in all domains.
During the early Proterozoic the Dharwar Craton area experienced HP amphibolite facies metamorphism with T 700-750 °C and P = 8-10 kbar (Raith & Schumacher, 2011). Corundum-fuchsite rock represent member of a suite of highly aluminous rocks which comprises fuchsite-bearing corundum-kyanite, kyanite-corundum and pure kyanite rocks, and probably forms an enclave of Cr-enriched high-alumina metasediments within a prominent unit of Bt+Grt+/-St+Ky schists and paragneisses. Such Cr-enriched argillaceous metasediments are typical components of the greenstone belts in the western Dharwar Craton, and reflect a significant contribution from ultramafic sources. Chemical attributes and the fine-grained nature of the corundum-fuchsite rocks suggest they formed through potassic-siliceous infiltration metasomatism from the kyanite-corundum members of the suite.

The extension of the Ms+Qz-shells along the planar fabric of the rock suggests they represent strain shadows developed around the rigid corundum grains during HP-amphibolite facies deformation. In such a case, dilation-controlled infiltration of potassic aqueous fluids into the extensional gashes offers a straightforward explanation of the coronal texture and mineral chemical features (Fig.2):

(1) development of a closed kyanite rim through an interface-controlled dissolution-precipitation reaction of corundum with dissolved silica (SiO2 aq),
(2) simultaneous precipitation of almost Cr-free muscovite from the fluid with coarsening of the adjacent fuchsite matrix,
(3) replacement of the kyanite rim by fuchsite through reaction with K+ and SiO2 aq.

Fig.2: Cartoon illustrating the synchronous formation of kanite+fuchsite-reaction rim and muscovite+quartz-strain shadow infill during infiltration of aqueous fluids into extensional gashes developed around a rigid corundum grain. Crn = Corundum; Fus = Fuchsite; Ky = kyanite; Ms = Muscovite; Qz = Quartz.

Fig.3: Corundum crystal (red) with blue-green kyanite rim in a fuchsite schist from Mercara region. From Silver enchantments.

Fig.4: Corundum crystal (red) with blue-green kyanite rim in a fuchsite schist from Mercara region. From Worth point.

Bibliography

• Raith, M. S. (2014): Ruby-fuchsite rocks from the Kodagu District, Southern India.
• Raith, M.M. & Schumacher, R. (2011). Corundum-kyanite-fuchsite rocks from the Kodagu District, Southern India. Joint Meeting DGK, DMG, OMG 'Crystals, Minerals and Materials' (Salzburg, Austria 2011), Abstract volume, 114-115, Oldenbourg Verlag-München.

Photo
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)	Corundum crystals with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. PPL image, 1x (Field of view = 9mm)
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)	Corundum crystals with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. PPL image, 1x (Field of view = 9mm)
Corundum crystals with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. PPL image, 1x (Field of view = 9mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 1x (Field of view = 9mm)
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)
Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)	Corundum crystals (twinned) with fine-grained fuchsite rim (ex-kyanite rim) and oval shaped shell of coarser-grained muscovite and quartz. XPL image, 2x (Field of view = 7mm)