QUARTZ

Quartz – SiO2 – is a very common tectosilicate, the second most abundant mineral in terrestrial crustal rocks after feldspars, important as rock-forming mineral in igneous, metamorphic and sedimentary rocks. It is very resistant to alteration and therefore it is the main constituent of sand.

Quartz crystals are usually combined forms; over 500 different combined shapes have been registered. The largest quartz crystal reached 6 m and 36 tons. Quartz is piezoelectric and pyroelectric; it can be triboluminescent and has rhombohedral partition. Usually contains Al and Fe replacing Si; for the balance of charges occur Fe, Na, Li and K in interstitials sites.

The polymorphs of silica are low temperature quartz (<573ºC), high temperature quartz (>573ºC), cristobalite, tridymite, moganite, coesite and stishovite.

1. Characteristics

Crystal system: Trigonal trapezohedral.

Color: Colorless to black with many colors in between these two extremes.

Habit: Massive, drusiform, microcrystalline, fine granular, hexagonal prisms with 2 rhombohedra, etc.

Cleavage: Rarely observable. {01-10} poor, {01-11} poor, {10-10} poor.

Tenacity: Brittle.

Twinning: Very common: Japan, Delfinado and Brazil laws, etc.

Fracture: Conchoidal.

Mohs Hardness: 7, but variable according to direction and shape.

Parting: May show rhombohedral partition.

Streak: White.

Lustre: Vitreous to dull.

Diaphaneity: Transparent.

Density (g/cm³): 2.59 – 2.63

 

2. Geology and Deposits

Quartz is extremely common in igneous, metamorphic and sedimentary rocks. Even in basic rocks, it is occasionally present. It is a common mineral in hydrothermal veins of any temperature (epithermal to alpine), characteristic of granitic granites and pegmatites. It is a rock forming mineral in sandstone and quartzite and less abundant in other rock types. It also occurs in hydrothermal metal deposits. It is common in carbonate rocks and is an important constituent in soils and sediments as a residual mineral.

In hydrothermal deposits of all types, quartz is one of the main gangue minerals and is usually older than sulfides. However, situations in which it occurs over several generations are not rare. Quartz is common filling cavities like vesicles in volcanic rocks and grows in clefts of the Alpine Type.

 

3. Mineral Associations

Quartz is so abundant that it is impossible to present a list of minerals that are associated with it. Very common is the association with feldspars (potassic feldspars and plagioclases), amphiboles (hornblende), micas (biotite, muscovite, chlorite, etc.), calcite, zeolites, epidote and other minerals. In ores, it is associated with gold and the many different sulfides (pyrite, chalcopyrite, arsenopyrite, etc.).

Quartz is less common or absent in silica-subsaturated igneous rocks and their metamorphic counterparts. It never occurs with feldspatoids such as nepheline, leucite, sodalite, cancrinite and others. In evaporitic sequences, its presence is accidental. Therefore, its association with halite, gypsum, anhydrite, ulexite, kernite and other soluble salts is not expected.

 

4. Transmitted Light Microscopy

Refraction indices:  ne: 1.553   no: 1.544

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colourless. It never has color or pleochroism. Even the colorful varieties are colorless under a microscope.

Relief: Low, like feldspars.

Cleavage: Poor cleavages in {10-11}, {01-11} and {10-10}, which are not visible under the microscope. In addition, it has a rhombohedral partition, which is also not visible under the microscope.

Habits: Usually anhedral grains. In certain volcanic rocks, such as rhyolites, it can present euhedral (idiomorphic) grains with hexagonal sections, which are paramorphic from beta quartz. In  hydrothermal veins it is often idiomorphic.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.009, resulting in middle 1st order colors: various shades of dark to light gray, white, at most straw-yellow.

Extinction: Parallel to the prism. As the quartz grains are usually anhedral, this determination is rarely possible. Quartz very often presents undulating extinction because it is subdivided into subgrains.

Elongation sign: ES(+) in relation to elongation, rarely observable because grains are almost always anhedral.

Twins: Very rare, usually does not present in a thin section.

Zoning: Very rare in thin sections.

CONVERGENT LIGHT

Character: U(+), but it is often anomalously biaxial; this is very common in igneous and metamorphic rocks.

2V angle: May have an anomalous 2V angle of up to 20º

Alterations: never alters. Among the common rock-forming minerals, quartz, tourmaline and zircon are the most stable minerals with respect to alteration.

May be confused with: several other minerals.

Feldspars have twins, cleavage, are biaxial and are usually somewhat altered to clay minerals, with a cloudy appearance, while quartz is clear. From feldspar without twins it can be distinguished by its U(+) character.

Scapolite, apatite, cordierite, tridymite, nepheline and beryl are very similar, but none of them is U(+).

 

Quartz 01: in PPL, a large quartz crystal surrounded below by slightly altered feldspars and above by other quartz grains. Without color, without cleavage and without any other diagnostic feature, it is not possible to identify quartz in PPL, it must be done in XPL.
Quartz 01: now in XPL, the same section as in the previous figure. Quartz never shows alteration and undulating extinction, as here, is very common.
In XPL, idiomorphic quartz crystal in a volcanic rock. Quartz is only euhedral in such situations or when it crystallizes in cavities and fractures.
In XPL, in a volcanic acidic rock, quartz with corroded edges.
Quartz and chalcedony filling a vesicle (gas bubble) in a volcanic rock (black in image). XPL.
Quartz in XPL (center of the image) in a plutonic rock, with feldspars (gray, stained – “cloudy”) and muscovite (orange and red colors).
Quartz in XPL (center of image) in a metamorphic rock, with feldspar (gray, stained) and muscovite (colorful).
Quartz in a metamorphic rock (quartzite), in very stretched grains. XPL.
Quartz grains in a sedimentary rock (sandstone). Quartz grains are cemented by chalcedony. XPL.
Unaltered quartz grains (very clear gray colors) alongside feldspar grains highly altered to clay minerals (kaolinite) and sericite (very fine-grained muscovite). XPL.
Spherulite in acid extrusive rock (rhyolite). These are aggregates of quartz and feldspar which tend to have a radial structure. XPL.
Quartz 2: In acidic extrusive rocks such as rhyolites, idiomorphic high-temperature quartz paramorphs are found. They are quartz crystals formed at T>573ºC, with a hexagonal prism terminated at both ends by rhombohedrons, simulating a bipyramid. By lowering the temperature, the quartz converts to low temperature quartz, keeping its original shape. In longitudinal section, the crystals are elongated, with parallel extinction and whitish interference colors.
Quartz 02: the same rock and setting of the previous figure, but here a basal section of a former beta-quartz, with the typical hexagonal outline.
Granophyric texture in an acidic volcanic rock. It is an intergrowth of quartz and feldspar, tending to radial, very common. XPL.
Quartz 03: Quartz crystals that grew in stages, with the limits of each stage preserved inside the crystals in the form of concentric figures (= rhombohedrons), as if they were tree trunk growth rings. It is called “phantom quartz” or “ghost quartz”. XPL.
Quartz 03: from the same rock of the previous image, single quartz crystal with the "ghosts" inside. It is necessary to observe carefully the crystals to see these features, which are rare.
Quartz can have several types of inclusions. In this case, the quartz is full of acicular crystals, probably from an amphibole. XPL.
Quartz 04: A quartz grain can be accidentally placed in a silica subsaturated rock, as here in a basaltic rock. It could be a xenolith during the magma rise or it could be a grain of sand thrown into the lava during the volcanic event. In this case, a ring (“corona”) of minerals such as clinopyroxene (augite), olivine and chlorite develops around the quartz grain. PPL.
Quartz 04: The same situation as of the previous figure, this time in XPL.
For more than a century, it was assumed that “Tiger's Eye” was a pseudomorph of quartz on crocidolite, which is a variety of riebeckite, a fibrous amphibole. It is not. New investigations (Heaney & Fisher, 2003) have shown that it is only quartz. The thin section is a little bit thicker than 30 microns because the quartz in these long, fibrous crystals offers great difficulty in grinding during the making of the thin section. XPL.
Fossil (silicified) wood, iin XPL. One can clearly see the plant cells with a well-preserved shape, entirely replaced by silica.In PPL it is colorless, but the cells can be seen.
In XPL, idiomorphic quartz crystals from a hydrothermal deposit. One crystal is twinned, a feature seldom observed in thin sections.
In XPL, graphic quartz. The "grains" without alteration (not cloudy) are quartz, immersed in a slightly altered feldspar (a bit darker).

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of quartz. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with quartz.

Sample preparation: Quartz polishing is relatively simple despite of its high hardness. It polishes better than many minerals of similar hardness.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, similar to feldspars. Lighter than the color of amphibole and pyroxene and much lighter than the color of biotite.

Pleochroism: No.

Reflectivity: ~4,5%

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Do not shows anisotropy.

Internal reflections: Abundant internal reflections between light gray and colorless, yellowish, can be multicolored.

May be confused with: many other light colored transparent minerals. When idiomorphic it is easier to recognize. The absence of cleavage and good polishing are diagnostic.

General Characteristics: 

Grain shape is often idiomorphic in hydrothermal ores.

Replacement of quartz with ores or calcite may occur.

Inclusions of very fine rutile needles can be seen very well in the polished sections.

Quartz 01: in PPL, a longitudinal section (long) and a basal section (hexagonal) of quartz with the characteristic gray reflection color. Cream colors are an ore mineral.
Quartz 01: the same section as in the previous image, now in XPL. Quartz has milky internal reflections and the ore mineral shows anisotropy between yellow and brown.
Quartz 02: : in PPL, quartz crystals in longitudinal sections. In yellowish, nickeline.
Quartz 02: the same section as in the previous image, now in XPL. Quartz shows milky internal reflections (may be multicolored). Nickeline shows the characteristic anisotropy colors (blue, lilac, etc.).
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