ZIRCON

Zircon is a common nesosilicate that occurs as an accessory mineral in many types of rock. They are usually very small crystals. It is an ore of zirconium (“zirconite”) obtained from placers and, when in large crystals, can be used as a gemstone. It is important for geochronology, employing uranium isotopes.

Zircon is part of the Zircon Group, with 5 more members and always contains impurities such as Hf (0.5-1%, being a solid solution that evolves into hafnon – HfSiO4, a rare mineral), U (up to several 1000 ppm), Th, Y/ETR, Pb, P, Fe, Al, H2O and others, in a total of fifty. When it contains radioactive elements, may be metamict in all stages from incipient to completely altered (opaque). It is thermoluminescent and cathodoluminescent. It can fluoresce in shades of yellow, golden-yellow and yellow-brown under ultraviolet light, which can help its identification in heavy sands.

It has 22 varieties, each with its own name, such as “starlite”, which is a blue zircon. Oriented intergrowths (epitaxys) with xenotime are common.

1. Characteristics

Crystal system: Tetragonal, bipiramidal ditetragonal.

Color: Reddish brown, yellow, green, blue, gray, colorless.

Habit: Usually tabular or prismatic with square basal sections. Irregular, massive grains.

Cleavage: {110} indistinct, {111} indistinct. It is not visible even in sample by hand or under a microscope.

Tenacity: Brittle.

Twinning: Rare.

Fracture: Conchoidal.

Mohs Hardness: 7.5

Parting: No.

Streak: White.

Lustre: Adamantine, vitreous, resinous.

Diaphaneity: Transparent.

Density (g/cm³): 4.7

 

2. Geology and Deposits

Zircon is a very common accessory mineral in granites, syenites, granodiorites, pegmatites, schists, gneisses and quartzites, where it occurs alone or as inclusions in biotite, hornblende and cordierite, always forming less than 1% of the rock mass. It also occurs as an accessory mineral in volcanic rocks such as andesites, trachytes and phonolites. In mafic pegmatites and carbonatites, it forms large crystals, up to 30 cm. Crystals of this type, several centimeters in length, can be purchased commercially, in stone stores. By sawing these crystals in half, it is possible to visualize spectacular zoning and produce thin sections and high-quality polished sections.

It has been found in some achondrite-type meteorites.

It is a common detrital mineral in clastic sediments and therefore occurrs in clastic sedimentary rocks. In this context, it may idiomorphic, contrasting with the well-rounded grains of other minerals. The yellow fluorescence of zircon helps to identify it in heavy (black) sands. It is, therefore, common in polished sections of placer concentrates from gold-magnetite-ilmenite-platinum-monazite-uraninite deposits, where it can provide important information about the origin of these sands.

 

3. Mineral Associations

Zircon occurs associated with many different minerals. Occurring in igneous rocks (and associated pegmatites), in metamorphic and sedimentary rocks, there are no unique and/or diagnostic associated minerals.

 

4. Transmitted Light Microscopy

Refraction indices:  ne: 1.925 – 1.961    no: 1.980 – 2.015

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. Weak pleochroism from colorless to brown in colored varieties.

May be cloudy or with concentric zonation or with color banding.

Relief: Very high, extreme.

Cleavage: {110} and {111} indistinct, usually not visible under a microscope.

Habits: Short prismatic with basal sections that tend to be square.

Forms subhedral, round to elliptical grains, often with a dark halo (brown, black) around them, but only when they occur within mafic minerals (biotite, amphibole, etc.). 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Very high birefringence, 3rd and 4th order, 0.036 to 0.062 (or 0.047 – 0.0550): intense, vivid and colorful in longitudinal sections.

In the basal sections, the colors tend to be dark, gray or yellowish.

Extinction: Parallel according to the elongation.

Elongation sign: ES(+)

Twins: Rare, according to {111} or {101} – literature diverges.

Zoning: Zircon can develop a spectacular zoning, formed by successive bands of different colors. Generally, however, the grains are too small to notice zoning, or only the edge is seen with a different color than the core. The older the rock (and the zircon), the more pronounced the zoning.

As the nucleous of the grains are very rich in U and Th, intense metamictization (isotropization) occurrs, which generates an increase in volume and can “explode” the outer layer of the grain.

CONVERGENT LIGHT

Character: U(+). It is usually quite difficult to get a good figure because the grains are so small. Basal sections (square to round), with interference colors between dark gray and yellowish, provide good figures as they approach the isotropy section of the optical indicator. Longitudinal sections (prismatic to elliptical), with intense interference colors, tend to provide figures (parallel to the optical axis) that are difficult to recognize. Zircon, when metamictic, is isotropic or biaxial.

2V angle: anomalous up to 10º is possible.

Alterations: does not alters easily. Concentrates in detrital (alluvial) placer deposits. If it contains radioactive elements in sufficient quantities, it can become metamict (isotropic).

May be confused with: it is easy to confuse zircon with epidote, monazite and titanite, especially when they occur in very small grains. The four occur together as accessory minerals in many rock types, have similar reliefs, similar interference colors, parallel or impossible-to-classify extinction (rounded grains), and often low-quality interference figures that are difficult to determine.

Epidote shows weak green/yellow pleochroism, has lower relief and is biaxial.

Monazite in PPL is normally colored in faint colors, has a sharp cleavage and is biaxial.

Titanite is brown, with lower interference colors and in CPL is often similar as in PPL.

Allanite usually shows pleochroism in brown to yellow colors.

Xenotime (YPO4) is also U(+), but shows pleochroism in yellow tones and has higher interference colors.

Apatite has a lower relief and gray interference color.

Cassiterite is not colorless.

Rutile is not colorless, is darker and has a higher refractive index.

 

Zircon 01: in PPL, zircon (colorless with dark halos) and biotite (brown). Zircon inclusions in biotite are very common and always develop such dark zones around the zircon grains.
Zircon 01: the same section as in the previous image, now in XPL. Zircon shows colorful interference colors.
In PPL, two colorless, high-relief zircon grains. The grain on the left is embedded in a felsic mineral (quartz) and does not develop a dark halo around it. The one on the right is included in a mafic mineral (biotite) and has a dark halo.
In XPL, zircons have strong and intense colors. The colors can be arranged in concentric bands, forming a very characteristic zonation.
In XPL, zircons have strong and intense colors. The colors can be arranged in concentric bands, forming a very characteristic zonation.
In XPL, zircons have strong and intense colors. The colors can be arranged in concentric bands, forming a very characteristic zonation.
In XPL, basal section of zircon. They typically tend to be square and show weak interference colors, between gray and yellowish. The interference figure in these sections is usually of good quality. In PPL, they are completely colorless, with the diagnostic high relief of zircon. Around it, quartz (white/grey) and biotite (colored).
In XPL, as in the previous image, basal section of zircon. Similar to basal sections of sillimanite, but sillimanite shows a diagonal cleavage in its basal sections and is not U(+).
In PPL, several aligned basal sections of zircon. In XPL they show the diagnostic strong interference colors of zircon.
In PPL, zoned zircon grain with fluorite on some of its sides. Such blue/violet colors are strongly diagnostic for fluorite, which is isotropic in XPL.
In PPL, some very small crystals with dark halos around them in biotite. Most probably are zircon grains, but monazite may be very similar.

5. Reflected Light Microscopy

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

Sample preparation: zircon polishing is simple as it acquires a good polish with ease. It is much easier to polish than common silicates (quartz, feldspars, micas, amphiboles and pyroxenes) and magnetite (which is much more difficult to polish). Polishing often makes it possible to recognize the zoning of the grains, as some areas are polished better and other areas are less polished, creating an easy-to-observe contrast.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, much lighter than quartz, feldspars, amphiboles, micas and apatite, for example. The color is very similar to the color of titanite.

Pleochroism: No.

Reflectivity: Low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy, if any, is completely masked by the intense and widespread internal reflections.

Internal reflections: Colorless grains show colorless reflections, while colored varieties (yellow, brown to red) show reflections in these colors. Colorless zones alternating with zones of varying color make zoning easy to observe.

May be confused with: zircon has three diagnostic characteristics under Reflected Light.

(1) Its reflection color is much lighter than that of the most common rock-forming minerals such as quartz, feldspars in general, amphiboles, pyroxenes and micas.

(2) Its reflectivity is much higher than the reflectivities of common silicates and is very similar to the reflectivity of titanite.

(3) Zircon acquires a much better polish than the minerals mentioned above; the polishing quality is as good as that of titanite and better than that of magnetite.

In XPL, on the other hand, its internal reflections can be confused with those of apatite (if colorless) and of sphalerite and titanite (when colored – brown).

Apatite has a similar reflection color, but is usually prismatic or occurs in hexagonal basal sections, always with very luminous white internal reflections.

Sphalerite does not show parallel extinction like zircon.

Zircon 01: in XPL and Transmitted Light(!), a thin section with magnetite (black), apatite (gray) and zircon (colorful). The next image is with Reflected Light.
Zircon 01: the same section as of the previous figure, now in PPL in Reflected Light. Even with a low quality polish, it is possible to observe that zircon has a very light color and the best polish of all. Magnetite is more reflective and lighter, but less polished (and isotropic to NC).
In PPL, biotite (internal reflections in brown) and quartz (internal reflections in white) are observed. Included in the biotite are tiny colorless grains of zircon, with black halos around them. The image is very similar to that observed in Transmitted Light in PPL.
In PPL, amphibole (green reflections) and quartz (white reflections) are visible. The amphibole contains a readily identifiable zircon inclusion, also with a black halo around it.
Zircon 02: in PPL, in the center of the image, zircon grain in sodalite-syenite (“Granito Azul Bahia”). Around it, cancrinite. On the periphery of the image, sodalite.
Zircon 02: the same section of the previous image, now in XPL. Sodalite with blue internal reflections, cancrinite with yellowish ones and zircon with a very nice zoning.
Zircon 03: giant brown zircon crystal seen under the stereomicroscope. Zoning is very evident: colorless band alternate with brown ones.
Zircon 03: section of the same giant crystal of the previous image, now in Reflected light in XPL. Color banding is evident; colors very similar than the ones of cassiterite.
In grain mount (placer deposit; black sands), in XPL, zircon with yellow internal reflections and very evident zoning. In these situations, it shows parallel extinction.
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