Page 98 - Laboratory Manual in Physical Geology

Shape of Broken Pieces
Number of Cleavages Name and Description (cleavage directions Illustration of
and Their Directions of How the Mineral Breaks Cleavage Directions
are numbered)

No cleavage No parallel broken surfaces; None
(fractures only) may have conchoidal (no cleavage)
fracture (like glass)
Quartz
Basal (book) cleavage
1
1 cleavage
"Books" that split apart
along flat sheets
Muscovite, biotite, chlorite (micas)
Orthoclase 90°
Prismatic cleavage (K-spar) 1
2 cleavages 2
intersect Elongated forms that fracture
at or near 90° along short rectangular
cross sections Plagioclase 86° & 94°,
pyroxene (augite) 87° & 93°

Prismatic cleavage
2 cleavages 1
do not intersect Elongated forms that fracture 2
at 90° along short parallelogram
cross sections
Amphibole (hornblende) 56° & 124°

1
3 cleavages Cubic cleavage
intersect 3 2
at 90° Shapes made of cubes and
parts of cubes
Halite, galena

Rhombohedral cleavage 1
3 cleavages 3 2
do not intersect Shapes made of
at 90°
rhombohedrons and parts
of rhombohedrons
Calcite and dolomite 75° & 105°
4 main cleavages
intersect at 71° and 109° Octahedral cleavage
to form octahedrons, 4 3
which split along hexagon- Shapes made of
shaped surfaces; may octahedrons and parts 1 2
have secondary
cleavages at 60° and 120° of octahedrons
Fluorite

Dodecahedral cleavage 2
6 cleavages
intersect at Shapes made of 6 1 3
60° and 120°
dodecahedrons and parts
of dodecahedrons 5 4
Sphalerite

FIGURE 3.12 Cleavage in minerals.

Mineral Properties, Identification, and Uses ■ 83