Mineral Deformation (from 306)
Goals
Learn type of crystal defects
Understand types of deformation
Understand relative ease of plastic deformation
Questions?
Crystal defects (Fig 4.52, p. 153)
Present in all crystals from growth, deformation
Point defects:
Schottky (missing ion/atom)
Frenkel (misplaced ion/atom)
Impurity:
Interstitial
Substitutional
Line defects:
Edge dislocation
Screw dislocation
Plane defects:
Tilt wall
Defects contribute to deformability
Without defects, minerals (& metals) would be hundreds of times stronger
Main types of deformation
solution
brittle
ductile
twinning (could be a kind of ductile)
granular
Solution, "Pressure Solution"
Mineral dissolves and maybe precipitates
Causes grains to dissolve in high-stress regions, and sometimes precipitate in lowstress regions
Occurs at low-T, diagenesis regime
Responsible for stylolites as well
Brittle
Micro-cracking
cataclastic flow
Ductile
Dislocation glide
different minerals have different ways that dislocations can slip
plane & direction = "slip system"
some slip systems are easier at some temperatures, some are easier at others.
quartz: basal slip at most T, prism slip at high T
forms ribbons
Twinning
common in calcite, plagioclase
requires other processes as well
Granular
grain boundary sliding
more prevalent with finer grain size
Relative ease of deformation
Calcite easiest, esp. at low-T
Quartz next
Feldspar hardest