Quartz and Its Look-Alikes

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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It's an unfortunate fact of life for beginning optical mineralogy students that a number of common minerals all have low relief and first-order gray interference colors. Here are some ways to tell them apart.

Quartz

QUARTZ.GIF (5611 bytes) Quartz in thin section is generally very clean looking without many inclusions. It frequently shows undulose extinction (A, right) or slightly yellowish interference colors (B).

 

Potassium Feldspar

MICROCL.GIF (10329 bytes) Potassium feldspar often shows good cleavage (A, left) and has a "dusty" appearance from tiny alteration inclusions (B). If "tartan" twinning is visible (A, right) the identification is certain. The inclusions often consist of sericite, or fine-grained muscovite, and show high interference colors (B)

 

Plagioclase Feldspar

PLAGIOCL.GIF (9889 bytes) Plagioclase feldspars also have good cleavage (A, left) and a dusty appearance from inclusions. They often show compositional zoning (A, right), but their most diagnostic feature is prominent lamellar twinning (B). The inclusions commonly turn out to be tiny crystals of epidote (C).

If your feldspar lacks twinning, it can be hard to tell them apart. Usually the dusty appearance, cleavage and alteration separate them from quartz. Sometimes the easiest approach is to use one of several staining methods.

Nepheline

Nepheline never occurs with quartz, unlike the other minerals shown here. It is most often found in silica poor igneous rocks that are rich in alkali feldspar. Nepheline is uniaxial, but in contrast to quartz is optically negative. It lacks cleavage, distinguishing it from the feldspars. Nepheline commonly alters to cancrinite, a Na- and Ca- bearing aluminosilicate. Cancrinite typically shows first order interference colors but the crystals are typically much larger than the very fine sericite  grains that form in K-feldspar, and are easily distinguishable.

Cordierite

CORDIERT.GIF (7125 bytes) Cordierite often shows dendritic "pinite" alteration along cracks (A, left). Yellow pleochroic halos around radioactive inclusions like zircon are common (B) as is yellowish alteration along cracks (C). Opaque inclusions are also common (D).

On the right cordierite is compared with plagioclase (A), microcline (B), perthite (C) and quartz (not labeled).


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Created 30 November 1999, Last Update 7 December 1999

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