Occurrence and Compositon
Quartz is a common and often essential mineral in a wide variety of sedimentary, metamorphic and igneous rocks. It is always nearly pure SiO2

Keys to identification are:colorless nature, first order white-gray interference colors, and undulatory extinction. It also lacks cleavage, twinning and alteration.

Important properties
·Color - colorless in thin section.
·Interference colors - generally first order white to gray but in some thin sections it may show a very weak first order yellow
·Undulatory extinctiion - wavy (undulatory) extinction is diagnostic; it develops because different parts of a grain may go extinct at slightly different orientations.
·Form - typically anhedral in igneous and metamorphic rocks, rarely euhedral prismatic.
·Relief and cleavage - low relief, may show disoriented fractures but never has cleavage.
·Interference figure - uniaxial (+).

Similar minerals
·Distinguishing quartz from plagioclase or alkali feldspars may be difficult. Feldspars, however, often show twinning, quartz does not. Additionally, feldspars may alter, giving a scuzzy or dirty appearance in PP light; quartz does not.
·Other similar minerals include apatite, cordierite, beryl and scapolite, but none of them are uniaxial (+) like quartz.

Quartz and Plagioclase in a Rhyolite Porphyry


Here we see large phenocrysts of quartz and plagioclase in a matarix of finer quartz and K-feldspar. The largest grain is quartz and appears to be split along a fracture in the XP view. Many other, smaller, quartz grains are also present. A single large grain of plagioclase can be identified by its zebra-stripe twins (XP). Note that in the PP view, the large quartz and feldspar phenocrysts appear to be much less altered than the finer grained groundmass.

Kyanite and Quartz, with Minor Muscovite

This veiw includes two kyanite grains/aggreagates, one a large blade showing cleavage and the other a patchy mass showing no cleavage. The kyanite is surrounded by quartz. The quartz is clear with low relief (compared to the kyanite) and low order interference colors, while the clear kyanite has high relief and interference colors range up to first order yellow. (In some thin sections, kyanite's interference color may be second order reds.) Note the undulatory (blotchy) extinction displayed by quartz. Several flakes of muscovite (clear with low relief in PP, and uper second order interference colors in XP) are just to the right of the kyanite. The muscovite cannot be distinguished in the PP view, but in the XP view its high-order interference colors stand out.

This view is about 1 mm across.

Quartz Adjacent to Biotite and Sillimanite in a Pelitic Gneiss

These photos are dominated by several large grains of quartz. They are clear (PP), slightly fractured in places, and display first order gray interference colors. Other, smaller, quartz crystals are scattered around the periphyry. A few small brown and tan (PP) biotite flakes are also present but are mostly lost in the XP view. At the bottom, near the lower left corner, the higher-relief blocky crystals are sillimanite. Although sillimanite typically forms needles, here we are looking down the axis of many of the needles so we see a square cross-section. Note that there is one diagonal cleavage cutting across the square sillimanite end sections.

This sample comes from Western Massachusetts; the field of view is about 1.5 mm.

Biotite, Quartz and K-feldspar (microcline) in a Granite

This view shows two large flakes of brownish green biotite and minor hornblende (darker smaller grains) near the left edge; the hornblende does not have a distinctive appearance. The biotite has a pebbly texture and micaceous cleavage. The right half of the view contains a large grain of clear to cloudy K-feldspar. The clear grains with low birefringence left of the K-feldspar are quartz, and minor plagioclase (feldspar) is in the extreme lower left. The feldspars and quartz are difficult to distinguish in the PP view, although the feldspars are a bit cloudier due to incipient alteration. In the XP view, poorly developed microcline twinning gives the K-feldspar an irregular mottled appearance, clearly distinguishing it from the quartz.

This sample is a granite from the Minnesota River Valley, Minnesota. The field of view is about 2 mm across.

Sillimanite, Quartz and Biotite in a Pelitic Gneiss

This sample contains needles/blades of sillimanite (somewhat fractured, running diagonally from lower right to upper left), quartz, biotite and magnetite. In the PP view, the sillimanite and quartz are colorless, the biotite is brown to tan, and the magnetite is opaque (black). In the XP view, the quartz shows low order white-gray interference colors, while the sillimanite shows higher order (upper second order blue-purple etc.) colors. The largest needle of sillimanite is about 2 mm long and has upper second order purple-red interference colors. Note the high relief of the sillimanite compared to quartz. Near the right hand side of the view, half way up, one sillimanite grain shows typical diagonal cleavage.

This sample comes from near Kazabazua, Quebec.

Quartz with Rutile and Muscovite in a Gneiss

Most of this view shows quartz. It is characterized by low relief (PP), undulatory extinction (XP) and low order interference colors (XP). The brownish, high-relief grains (PP) are rutile. Rutile has very high birefringence but the color of the grain is so strong that its interference colors don't show (XP). A number of small flakes of muscovite are also present. They are clear and can barely be distinguished from the quartz in the PP view. In XP they are marked by their flakey shape and high second-order interference colors.

This sample comes from New Ogilby, California. The field of view is about 1.5 mm.

Quartz, Plagioclase and Orthoclase in a Quartz Monzonite from near Garfield, Colorado

It can be difficult to distinguish quartz and the different feldspar varieties from each other. Here, the quartz can be seen to be fresher (unaltered) in the PP view; this is often a key distinction. One large quartz grain is near the center of the view, and two others are near the left hand side. The large plagioclase grain in the upper right is twinned in an unusual way, but the small plagioclase down and to the left of the quartz shows more classic polysynthetic twinning (appearing as vague black and gray stripes in XP). Most of the bottom part of the thin section is orthoclase -- here it is near extinction (XP). A single, high relief, sphene can be seen near the lower left corner (PP), and several small grains of magnetite (opaque; black in both views) are present.

The field of view is about 2.5 mm.