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Natural corundum minerals: ground and polished white corundum fine powder, brown corundum grit, and black corundum abrasives—hardness, content, and applications.

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2023-11-18

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Among natural corundum minerals, white fused alumina micron powder, brown fused alumina grain‑size abrasive, and black fused alumina abrasives rank just below diamond abrasives in hardness. These materials—white fused alumina micron powder, brown fused alumina grain‑size abrasive, and black fused alumina (Al₂O₃)—belong to the trigonal crystal system; their crystals exhibit a transition from ionic to covalent bonding and possess a relatively compact structure. Individual crystals typically occur in drumstick or columnar forms, while aggregates appear as granular or dense massive textures. The color is generally bluish‑gray or yellowish‑gray, with iron‑bearing varieties appearing black. They display a vitreous luster, have a Mohs hardness of 9, and lack cleavage. Twinning often gives rise to cleavage planes along {0001} or {101}. Their density ranges from 3.95 to 4.10 g/cm³, and they are chemically stable. When chromium imparts a red hue, the corresponding white fused alumina micron powder, brown fused alumina grain‑size abrasive, and black fused alumina are referred to as ruby; when titanium produces a blue color, they are called sapphire.
　　Among natural abrasive materials such as white corundum micro‑powder, brown corundum grit, and black corundum abrasives, when they contain impurities like magnetite, hematite, and quartz and occur as granular aggregates resembling iron ores in appearance, they are referred to as white corundum micro‑powder, brown corundum grit, and black corundum abrasive sand (also known as natural emery). These materials typically comprise about 60% white corundum micro‑powder, brown corundum grit, and black corundum abrasive, usually exhibiting bluish‑gray or black hues, with a density of 2.7–4.3 g/cm³ and a Mohs hardness of 7–9. White corundum micro‑powder, brown corundum grit, and black corundum abrasive can form during magmatic, contact metamorphic, and regional metamorphic processes. When deposits or rocks containing these abrasives—whether of magmatic, contact‑metamorphic, or regional‑metamorphic origin—are subjected to weathering, the abrasive components often become concentrated in placer deposits. Major domestic sources include Liuhe in Jiangsu Province and Yingshan in Hubei Province. This mineral resource is abundant in many regions worldwide. As abrasives, white corundum micro‑powder, brown corundum grit, and black corundum abrasive sands are commonly used to manufacture grinding wheels, oilstones, sandpaper, sandcloth, as well as fine powders and polishing pastes.

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