Page 218 - Laboratory Manual in Physical Geology

■ Slaty rock cleavage — a very flat foliation (resembling ■ Phyllitic texture — a wavy and/or wrinkled foliation
mineral cleavage) developed along flat, parallel, closely of fine-grained platy minerals (mainly muscovite or
spaced shear planes (microscopic faults) in tightly folded chlorite crystals) that gives the rock a satiny or metal-
clay- or mica-rich rocks ( FIGURE 7.3 ). Rocks with excel- lic luster. Rocks with phyllite texture are called phyllite
lent slaty cleavage are called slate ( FIGURE 7.7 ), which ( FIGURE 7.8 ). The phyllite texture is normally developed

is used to make roofing shingles and classroom black- oblique or perpendicular to a weak slaty cleavage, and it
boards. The flat surface of a blackboard or sheet of roof- is a product of intermediate-grade metamorphism.
ing slate is a slaty cleavage surface.

Slaty cleavage surfaces

Clay minerals have been
changed to chlorite and
muscovite mica, which are
weakly foliated (aligned).

Edges of
well-developed
slaty cleavage
(shear planes) along
which the rock
A. Hand samples, 1 prefers to break B. Side view, 30

FIGURE 7.7 Slate. Slate is a foliated metamorphic rock with dull luster, excellent slaty cleavage, and no visible grains. Slate forms from

low-grade metamorphism of mudstone (shale, claystone). Clay minerals of the mudstone parent rock change to foliated chlorite and muscovite

mineral crystals. Slate splits into hard, flat sheets (usually less than 1 cm thick) along its well-developed slaty cleavage ( FIGURE 7.3 ). It is used to

make roofing shingles and classroom blackboards.
A. Top view, 1
Weakly developed
slaty cleavage
Well-formed muscovite crystals
cause development of a strong
Foliation surfaces
foliation along which the rock
prefers to break.

Weakly developed
slaty cleavage
(poorly developed
compared to slate)

B. Side view, 30

FIGURE 7.8 Phyllite. Phyllite is a foliated, fine-grained metamorphic rock, with a satiny, green, silver, or brassy metallic luster and a wavy

foliation with a wrinkled appearance ( phyllite texture ). Phyllite forms from low-grade metamorphism of mudstone (shale, claystone), slate, or
other rocks rich in clay, chlorite, or mica. When the very fine-grained mineral crystals of clay, chlorite, or muscovite in dull mudstone or slate are
metamorphosed to form the phyllite, they become recrystallized to larger sizes and are aligned into a wavy and/or wrinkled foliation ( phyllite
texture ) that is satiny or metallic. This is the wavy foliation along which phyllite breaks. Slaty cleavage may be poorly developed. It is not as obvious
as the wavy and/or wrinkled foliation surfaces. The phyllite grade of metamorphism is between the low grade that produces slate ( FIGURE 7.7 )
and the intermediate grade that produces schist ( FIGURE 7.9 ).
Metamorphic Rocks, Processes, and Resources ■ 193