Brief introduction to slaty cleavage

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Brief introduction to slaty cleavage

• Stone-Wang 01003122

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Contents

• ROCK CLEAVAGE
• CONCEPT OF SLATY CLEAVAGE
• BRIEF INTRODUCTION TO THE FORMATION OF
  SLATY CLEAVAGE

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Before introducing slaty cleavage, I will remind
you know something about rock cleavage.
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• Cleavage is defined as fine planar secondary
  structures produced deformation.

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• The term cleavage refers to the fissility of a
  rock, which allows it to be split along a set of
  foliation planes, and embraces structures of
  various origins, formed at low temperature.

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Cleavages are divided into three major types

• (a) fracture cleavage
• (b) slaty cleavage
• (c)crenulation cleavage

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The following part is emphasized on slaty
cleavage.
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Conception

• Slaty cleavage is encountered in fine-grained
  weakly metamorphosed rocks and is composed of
  parallel arrangements of very fine foliated
  minerals such as mica and clays.

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• The nature of internal changes in the rock that
  have produced this penetrative fissility is not
  usually obvious at out crop or in hand specimen.
  Under the microscope, however, the nature of the
  cleavage becomes much clearer.

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• The film is especially well developed around
  large rigid crystals and rock fragments, which
  can provide shelter from directed stress.

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BRIEF INTRODUCTION TO THE FORMATION OF
SLATY CLEAVAGE

• Mechanical rotation
• Recrystallization
• Pressure solution

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• At the early time, scientists explained the
  alignment of such mineral was due to rigid body
  rotation of these minerals during distortion of
  host rock.

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• The direction of crystal growth is the direction
  of extension, perpendicular to the direction so
  stress regardless of the attitude of bedding cut
  by the cleavage.

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• Significant improvement in explaining the
  formation of slaty cleavage has been made after
  the role of pressure solution in crystal
  deformation was realized.

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The densely packed concentrations of micas and
carbonaceous matter in cleavage seams represent
the accumulations of less soluble residue of the
original host rock.
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• The strong preferred orientation of micas in the
  M-domains may be the natural result of the
  rotation of micas in response to the stress. But
  some one pointed out that the preferred
  orientation of micas in slaty cleavage could take
  place without grain rotation. The general
  mechanism is illustrated in the FIGURE.

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It shows the fossil of graptolite are deformed in
certain conditions.
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Thats all

• Thank you for listening