Right-Angle Cleavage in a Pegmatite Specimen
During a laboratory practical, a student examines a pale, glassy mineral separated from a granite pegmatite. The specimen has a hardness of about 6.5: it scratches a steel file but is not scratched by a copper coin, and it leaves a white streak. Two cleavage directions meet at nearly 90 degrees, producing blocky fragments; no striations are visible on the cleavage faces. Specific gravity is 2.62. The mineral is vitreous to pearly, does not react with dilute hydrochloric acid, and is associated with quartz, muscovite, and pink potassium feldspar. The practical asks the student to choose among quartz, calcite, orthoclase, and plagioclase using a dichotomous key that requires at least two independent properties for each elimination. A classmate argues that the specimen must be quartz because it is pale and glassy. The instructor asks for a defensible identification that acknowledges any remaining uncertainty rather than forcing a single name.
1. Identify the mineral as precisely as the evidence allows and eliminate each alternative using at least two independent properties.
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Feldspar is the best fit, most likely plagioclase (albite–oligoclase), though orthoclase cannot be excluded without staining or optical work. Hardness ~6.5 is too low for quartz (7) and too high for calcite (3); the two cleavages at ~90° exclude quartz (no cleavage, conchoidal fracture) and calcite (three rhombohedral cleavages). No HCl reaction rules out calcite. White streak and 2.62 specific gravity are consistent with feldspar. Orthoclase shares hardness and cleavage, but the pink K-feldspar already in the pegmatite makes a second, pale feldspar more likely plagioclase; the absence of striations is not decisive because striations are best seen on albite twinning under crossed polars. Calcite is further excluded by the pegmatite association. The defensible answer is “plagioclase feldspar, probable”-not quartz.
2. Evaluate the classmate’s “pale and glassy = quartz” reasoning and describe one quick test that would strengthen the identification.
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Pale color and vitreous luster are not diagnostic: quartz, plagioclase, and orthoclase can all be white or colorless and glassy. Quartz’s key physical property is the absence of cleavage; it breaks conchoidally, so a fresh surface shows curved, shell-like fracture rather than flat cleavage faces. Rotating the specimen under a hand lens, the student should look for two sets of parallel planar surfaces meeting at ~90°-those establish feldspar cleavage and eliminate quartz. A hardness test against glass is less decisive: both quartz and feldspar can scratch glass, although quartz scratches more readily. A simple staining test, such as cobaltinitrite for K-feldspar, or a thin-section check for albite twinning would settle orthoclase versus plagioclase. The classmate’s argument relies on one property and ignores cleavage, so it is not a valid identification.
3. What does the presence of this feldspar with quartz, muscovite, and K-feldspar imply about the pegmatite’s parent magma and cooling history?
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The assemblage quartz + alkali feldspar + plagioclase + muscovite is granitic and felsic. Muscovite requires abundant aluminum and water, and its presence with quartz indicates a silica-rich, hydrous melt rather than a dry mafic one. A pegmatite’s very coarse crystals imply slow cooling and a water-rich residual melt that lowered viscosity and promoted large crystal growth. If the plagioclase is sodic (albite–oligoclase), the magma was not unusually calcium-rich; a calcic plagioclase would point to a more intermediate composition or contamination. The coexistence of two feldspars suggests crystallization at temperatures below the alkali-feldspar solvus, and muscovite stability implies relatively low temperature and/or high water pressure. The sample therefore supports a late-stage, hydrous granitic pegmatite, not a quartz vein or a carbonate rock.