Page 100 - Laboratory Manual in Physical Geology

Plagioclase Pink K-feldspar (orthoclase) White K-feldspar (orthoclase)
Fracture Cleavage
surfaces plane with
parallel,
86° 90° closely spaced
Right-angle striations
cleavage Fracture
surfaces at
Cleavage broken ends
planes of crystals
with no Cleavage plane
striations with subparallel
color variation
bands from
exsolution
lamellae
Cleavage
plane with
parallel,
closely spaced Cleavage surface
striations with striations
Right-angle
94° cleavage
90°
A. Plagioclase B. Pink K-feldspar (orthoclase) C. Rock comprised of plagioclase crystals

FIGURE 3.15 Common feldspars. Note how the cleavage goniometer can be used to distinguish potassium feldspar (K-spar, orthoclase)

from plagioclase. The K-spar or orthoclase (Greek, ortho —right angle and clase —break) has perfect right-angle (90º) cleavage. Plagioclase
(Greek, plagio —oblique angle and clase —break) does not. A. Plagioclase often exhibits hairline striations on some of its cleavage surfaces.
They are caused by twinning : microscopic intergrowths between symmetrically-paired microcrystalline portions of the larger crystal. B. K-par
(orthoclase) crystals may have intergrowths of thin, discontinuous, exsolution lamellae . They are actually microscopic layers of plagioclase that
form as the K-spar cools, like fat separates from soup when it is refrigerated. C. Hand sample of a rock that is an aggregate of intergrown pla-
gioclase mineral crystals. Individual mineral crystals are discernible within the rock, particularly the cleavage surfaces that have characteristic
hairline striations.

another carbonate mineral that resembles calcite, but it that is itself a natural magnet. It will attract steel paper-
will fizz in dilute HCl only if the mineral is first powdered. clips. Some other minerals may also be weakly attracted to
(It can be powdered for this test by simply scratching the a magnet (e.g., hematite, bornite, and pyrrhotite).
mineral’s surface with the tip of a rock pick, pocket knife,
or nail.) If HCl is not available, then undiluted vinegar can Specific Gravity (SG). Density is a measure of an object’s

be used for the acid test. It contains acetic acid (but the mass (weighed in grams, g) divided by its volume (in
3
effervescence will be much less violent). cubic centimeters, cm ). Specific gravity is the ratio of the
Striations are straight “hairline” grooves on the density of a substance divided by the density of water. Since
3

cleavage surfaces or crystal faces of some minerals. This water has a density of 1 g>cm and the units cancel out,
can be helpful in mineral identification. For example, specific gravity is the same number as density but without
you can use the striations of plagioclase feldspar any units. For example, the mineral quartz has a density of
3

( FIGURE 3.15A ) to distinguish it from potassium feldspar 2.65 g>cm so its specific gravity is 2.65 (i.e., SG = 2.65 ).

(K-feldspar, FIGURE 3.15B ). Plagioclase feldspar has faint Hefting is an easy way to judge the specific gravity of one

hairline striations on surfaces of one of its two cleavage mineral relative to another. This is done by holding a piece
directions. In contrast, K- feldspar (orthoclase) sometimes of the first mineral in one hand and holding an equal-sized
has internal exsolution lamellae, which are faint streaks of piece of the second mineral in your other hand. Feel the
plagioclase that grew inside of it. difference in weight between the two samples (i.e., heft the
Magnetism influences some minerals, such as mag- samples). The sample that feels heavier has a higher specific
netite. The test is simple: check to see if the mineral is gravity than the other. Most metallic minerals have higher
attracted to a magnet. Lodestone is a variety of magnetite specific gravities than nonmetallic minerals.
Mineral Properties, Identification, and Uses ■ 85