THE MINERAL ORPIMENT


Orpiment is a rare mineral that usually forms with realgar. In fact the two minerals are almost always together. Crystals of orpiment are extremely rare as it usually forms masses and crusts. The masses are sometimes transparent to a degree and have a gemmy quality to them. The yellow color is special to orpiment and can be confused only with a few other minerals. Orpiment is derived from the latin auripigmentum, or golden pigment. Its use as a dye or pigment is limited due to its instability. Over time, orpiment will deteriorate into a powder. The process takes a long time, but exposure to light will accelerate it. Specimens should be stored in dark, enclosed containers.

PHYSICAL CHARACTERISTICS:

  • Color is orange-yellow to yellow.
  • Luster is resinous to pearly
  • Transparency crystals are translucent to transparent.
  • Crystal System Monoclinic; 2/m
  • Crystal Habit: is usually foliated or earthy masses and crusts, also fiberous and as small tabular crystals that appear orthorhombic.
  • Cleavage is perfect in one direction producing flexible, non-elastic flakes.
  • Fracture is flaky.
  • Hardness is 1.5-2.
  • Specific Gravity is 3.5
  • Streak is yellow
  • Associated Minerals realgar, calcite, stibnite, barite and gypsum.
  • Other Characteristics: orpiment is unstable in light; specimens should be stored in complete darkness. Also usually has a distinct odor similar to sulfur, but is due to the arsenic.
  • Notable Occurances Romania; Peru; Japan; Mercur, Utah, USA and Australia.
  • Best Field Indicators are crystal habit, cleavage, odor and color.
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