The existence of gem-quality corundum on Chimwadzulu Hill in southern Malawi was first reported in 1958 (K. Bloomfield, "The Chimwadzulu Hill ultrabasic body," Transactions of the Geological Society of South Africa, Vol. 61). Chimwadzulu Hill is west-northwest of Ntcheu, approximately 5 km from the border with Mozambique. The area mined is on the summit of the hill, at an altitude of about 1,525 m. The eluvial deposits have been worked sporadically since the early 1960s. Early mining efforts recovered mainly sapphire, which was predominantly in blue, green, and yellow hues. Most of the sapphires were not highly saturated, and therefore required heat treatment.

In the 1980s, mining was conducted—with German technical assistance—by a quasi-government organization known as the Malawi Development Corp. Workings were concentrated on the crown of the hill, with some good-quality ruby recovered from one area, but with no discernible continuity or trend in the mineralization. The large quantities of sapphire that entered the market from other locations throughout the world eventually rendered this operation uneconomic, and mining was terminated in 1986. Heavy industry was largely discouraged during the Presidency of Hastings Banda (1964–1994), and no further attempts at organized mining were made in the eight years following mine closure.

In September 1994, Mineral Exploration Pvt. Ltd. (Minex) was granted a license to mine on Chimwadzulu Hill and to explore contiguous areas. Willard International, a Panamanian company with head offices in London, is the mine operator and 85% owner of Minex. Geologic exploration and limited mining efforts initiated late in 1995 resulted in the recovery of an estimated 100+ kg of sapphire similar to that described above; rubies were found occasionally, but there was still no identifiable trend to their recovery. However, the subsequent digging of test pits led to the discovery of a significant ruby trend in mid-1997.

Chimwadzulu Hill is composed of an ultramafic body that is hosted by metasedimentary rocks (i.e., gneisses and schists; see the Bloomfield reference cited above, and K. Bloomfield and M. S. Garson, 1965, The Geology of the Kirk Range—Lisungwe Valley Area, published by The Government Printer, Zomba, Malawi). The intrusive was subsequently metamorphosed, resulting in the formation of serpentinized peridotite and amphibolite. It dips northeast, and has a surface exposure of about 1 km in diameter. Over much of Chimwadzulu Hill, the bedrock has been deeply weathered and altered to an aggregate of deep red, friable, iron-rich, porous clay. In various places, the weathered bedrock is overlain by this residual soil. Corundum is recovered from these soils—particularly where this gem mineral is abundant in the underlying bedrock—and from the weathered bedrock itself.

The systematic test pits revealed that the majority of the chromium-bearing corundum (ruby or orange, pink, or purple sapphire) is localized within areas of weathered bedrock that are usually 20 m or less in lateral extent. Approximately 100 of these potential mining targets have been tentatively identified. In limited portions of these "hot spots," concentrations of chromium-bearing corundum can be very high; one pit 2 × 2 × 2 m yielded 7.6 kg of corundum. Mining of the present site on Chimwadzulu Hill is continuing, as is exploration of other corundum occurrences to the north.

The chromium-bearing corundum forms short columnar to tabular hexagonal crystals or crystal fragments. The basal pinacoids (0001) are very well developed, as are the prism faces (1120). On some of the more tabular crystals, the rhombohedral face (1011) is prominent. Typical dimensions of complete crystals are 1–3 cm across the basal pinacoid and 0.5–2.0 cm along the c-axis. Although the crystals generally are significantly fractured, the material between fractures is usually of very high clarity. The color of the chromium-bearing corundum ranges from orange through red, to purplish red, almost reminiscent of rhodolite garnet. In terms of tone, recovered material ranges from pale pink through a very deep red. A few crystals exhibit orange and red banding, or orange and purplish red banding, parallel to the basal pinacoid.

According to David Hargreaves of Willard International, approximately 80 kg of chromium-bearing corundum have been recovered since September 1997. Cutting and marketing of the polished stones is arranged by The Silurian Co. of London. Faceted stones of less than 1 ct are plentiful, and stones of 1–5 ct are not uncommon (see figure). A number of very fine gemstones have been cut, the largest of which weighs 16 ct. The clarity of these stones renders heat treatment unnecessary.

John L. Emmett
Crystal Chemistry
Brush Prairie, Washington

Photo GIA and Tino Hammid