Identification of Minerals

Professor John J. Renton

Outline lecture notes [JA]**

  • Identity = composition and structure
  • Composition easily determined with modern instruments
  • Structure harder/more expensive
  • Hand identification possible but only as good as the person doing it.
  • Examine thin sections under microscope - diffraction, bending, colour, interference
  • X-ray diffraction gives 3D structure (almost unique fingerprint)
  • in field, use streak test, cleavage, fracture, acid test
  • Mohs' scale of hardness from 1 (talc) to 10 (diamond)

**more detailed lecture notes available to members on request

Further information

As most of us don't carry 10 minerals to test hardness, here is a rough practical way of identifying mineral hardness, kindly passed on by HL:-

  • minerals grade 1 - can be scratched by fingernail and feel greasy in the hand - eg talc
  • minerals grade 2 - can be scratched by fingernail but don't feel greasy
  • minerals up to grade 3 - can be scratched by copper coin or piece of copper wire
  • minerals up to grade 5 - can be scratched by pocket knife (hardness of ~6) - a good knife will even scratch quartz
  • minerals of hardness greater than 6 will scratch window panes [be careful where you try this]
  • minerals of grades 8-10 - cannot be be scratched by file and can be used in making sparks

N.B. This is only a rough guide, as the hardness of the same mineral may vary in different conditions. Sometimes, soft aggregates are intergrown with harder minerals which may make the aggregate seem harder than it is. Chalk may be cut by a fingernail even though it has a hardness of 3 whilst a compact piece of gypsum (talc) - grade 1 may be scratched by a fingernail with difficulty.