Identify the naturally occurring aluminum oxide mineral that usually forms hexagonal barrel-shaped prisms.

The correct answer is Corundum.

Key Points

• Corundum is a naturally occurring aluminum oxide mineral (Al2O3).
• It commonly forms hexagonal barrel-shaped prisms, which can be terminated by flat surfaces.
• Corundum is renowned for its hardness, scoring a 9 on the Mohs scale of mineral hardness.
• This mineral is the primary source of gem-quality sapphires and rubies.
• Corundum is typically found in metamorphic rocks such as schist and marble.

Additional Information

• Mohs Scale of Mineral Hardness:
  • It is a qualitative ordinal scale characterizing scratch resistance of various minerals.
  • Devised by the German geologist and mineralogist Friedrich Mohs in 1812.
  • It ranges from 1 (talc) to 10 (diamond).
• Sapphires and Rubies:
  • Both are varieties of corundum distinguished by color.
  • Rubies are red due to the presence of chromium.
  • Sapphires can be blue, yellow, or even colorless.
• Metamorphic Rocks:
  • Formed through the transformation of pre-existing rocks under high pressure and temperature.
  • Examples include schist, marble, and gneiss.
• Industrial Uses of Corundum:
  • Used as an abrasive due to its hardness.
  • Employed in cutting tools and grinding wheels.
  • Also utilized in the manufacture of refractory materials.