Page 99 - Laboratory Manual in Physical Geology

Notice how this muscovite mica splits
Cleavage
surface apart into thin, transparent, flexible sheets Cleavage
along its excellent cleavage surfaces
surface
Cleavage
surface

Fracture
Fracture
surface 87°
surface
87°
Crystal
Cleavage
surface
Crystal
fragment

A: Pyroxenes (like augite) have two prominent
FIGURE 3.13 Cleavage in mica. Mica is a group of silicate cleavage directions that intersect at nearly right
minerals that form very reflective (vitreous) tabular crystals with angles (87° and 93°). They form prismatic crystals
one excellent cleavage direction. The crystals split easily into with a squarish cross section. The crystals break
thin sheets, like pages of a book. This is called book cleavage . into blocky fragments.
Muscovite mica is usually silvery brown in color. Biotite mica is
always black.

Cleavage
surface Cleavage
surface
cleavage directions. The two groups of minerals are some-
times difficult to distinguish, so some people identify Crystal
them collectively as pyriboles . However, pyroxenes can be
distinguished from amphiboles on the basis of their cleav-
age. The two cleavages of pyroxenes intersect at 87° and
93°, nearly at right angles ( FIGURE 3.14A ). The two cleav- Crystal

ages of amphiboles intersect at angles of 56° and 124° fragment

( FIGURE 3.14B ). These angles can be measured in hand
samples using the cleavage goniometer from GeoTools
Sheet 1 at the back of this manual . Notice how a green
cleavage goniometer was used to measure angles between Fracture
Fracture
surface
cleavage directions in FIGURE 3.14 . surface

Cleavage Direction in Feldspars. Feldspars have two
excellent to good cleavage directions, plus uneven fracture 56°
56°
( FIGURE 3.15 ). The cleavage goniometer from GeoTools

Sheet 1 can be used to distinguish potassium feldspar
(orthoclase) from plagioclase ( FIGURE 3.15 ).

B: Amphiboles (like hornblende) have two prominent
cleavage directions that intersect at 56° and 124°.
Other Properties. There are additional mineral proper- They form more blade-like crystals with a
ties, too numerous to review here. However, the follow- six-sided diamond-shaped cross section and
break into blade-like fragments.
ing other properties are typical of specific minerals or
mineral groups :
FIGURE 3.14 Cleavage in pyroxenes and amphiboles.
Tenacity is the manner in which a substance resists Pyroxenes and amphiboles are two groups of dark colored silicate
breakage. Terms used to describe mineral tenacity include minerals with many similar properties. The main feature that
brittle (shatters like glass), malleable (like modeling clay distinguishes them is their cleavage.
or gold; can be hammered or bent permanently into new
shapes), elastic or flexible (like a plastic comb; bends but
returns to its original shape), and sectile (can be carved

with a knife). including carbonate, CO ) will effervesce (“fizz”) when a
3
Reaction to acid differs among minerals. Cool, dilute drop of such dilute HCl is applied to one of their freshly
hydrochloric acid (1–3% HCl) applied from a drop- exposed surfaces ( FIGURE 3.16 ). Calcite 1CaCO 2 is the

3
per bottle is a common “acid test.” All of the so-called most commonly encountered carbonate mineral and
carbonate minerals (minerals with a chemical composition effervesces in the acid test. Dolomite 3Ca,Mg1CO 2 4 is

3 2
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