Page 97 - Laboratory Manual in Physical Geology

Light rays Cleavage Direction. Cleavage planes are parallel surfaces
of weak chemical bonding (attraction) between repeating
parallel layers of atoms in a crystal, and more than one set
of cleavage planes can be present in a crystal. Each different
set has an orientation relative to the crystalline structure
and is referred to as a cleavage direction ( FIGURE 3.12 ). For

A. Cleavage excellent or perfect (large, parallel, flat surfaces) example, muscovite ( FIGURE 3.13 ) has one excellent cleav-
age direction and splits apart like pages of a book (book

Light rays cleavage). Galena ( FIGURE 3.2 ) breaks into small cubes and
shapes made of cubes, so it has three cleavage directions
developed at right angles to one another. This is called
cubic cleavage ( FIGURE 3.12 ).

Cleavage Direction in Pyriboles. Minerals of the

pyroxene (e.g., augite) and amphibole (e.g., hornblende)
B. Cleavage good or imperfect (small, parallel, flat, groups generally are both dark-colored (dark green to
stair-like surfaces)
black), opaque, nonmetallic minerals that have two good
Light rays

A: Pure quartz (var. rock crystal) is colorless,
transparent, nonmetalic, and has conchoidal
fracture (like glass).

C. Cleavage poor (a few small, flat surfaces difficult to detect)
Light rays

Conchoidal: Uneven: Hackly: Splintery: Fibrous:
smooth rough breaks splinters separates
curved irregular along like wood into soft
fracture fracture jagged fibers, like
surfaces, surfaces surfaces cloth
like glass like broken
metal
D. Fractures (broken surfaces lacking cleavage planes) (x1)

FIGURE 3.10 Recognizing cleavage and fracture. Illustrated (x1)
cross sections of mineral samples to show degrees of development
of cleavage—the tendency for a mineral to break along one or
more sets of parallel, planar, reflective surfaces called cleavage
planes . If a broken piece of a mineral crystal is rotated in bright light,
its cleavage planes will be revealed by periodic flashes of light from
one large, or many small, flat parallel surfaces. If no such reflective
flashes of light occur, then the mineral sample has no cleavage.
Fracture refers to any break in a mineral that does not occur along
a cleavage plane. Therefore, fracture surfaces are normally not flat
and they never occur in parallel sets.

Fracture refers to any break in a mineral that does not
occur along a cleavage plane. Therefore, fracture surfaces B: Milky quartz forms when the quartz has
are normally not flat and they never occur in parallel sets. microscopc fluid inclusions, usually water. It has
an irregular (rough, uneven) fracture.
Fracture can be described as uneven (rough and irregular,
like the milky quartz in FIGURE 3.11B ), splintery (like splin-

tered wood), or hackly (having jagged edges, like broken FIGURE 3.11 Fracture in quartz—SiO 2 (silicon dioxide).
These hand samples are broken pieces of quartz mineral crystals so
metal). Pure quartz ( FIGURE 3.11A ) and mineraloids like no crystal faces are present. Note the absence of cleavage and the

opal ( FIGURE 3.3 ) tend to fracture like glass—along ribbed, presence of conchoidal (like glass) to uneven fracture.

smoothly curved surfaces called conchoidal fractures .
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