GARNET

Garnet – A3B2(SiO4)3   A=(Mg,Mn,Ca,Fe), B=(Al,Cr,Fe) – is a very common nesosilicate, which can be found in igneous, metamorphic and sedimentary rocks. It is an important industrial mineral with many applications, mainly as an abrasive.

“Garnet” is actually not a mineral, but just a generic term referring to members of the Garnet Group: pyrope – Mg3Al2(SiO4)3, almandine – Fe2+3Al2(SiO4)3, spessartine – Mn2+3Al2(SiO4)3, uvarovite – Ca3Cr2(SiO4)3, hydrogrossular – [Ca3Al2(SiO4)3-x(H4O4)x], grossular – Ca3Al2(SiO4)3 and andradite – Ca3Fe3+2(SiO4)3. There is a solid solution series between grossular and andradite, and for unanalyzed members of this series the term “grandite” is used. Garnets usually are isotropic, but can be anisotropic (“Cesare B. et al. 2019, Garnet, the archetypal cubic mineral, grows tetragonal”). It is very difficult to differentiate the members of the Garnet Group just by their optical characteristics.

Each of the garnets has some varieties, usually based on the contents of other elements. Inclusions of rutile, riebeckite and other minerals may occur.

1. Characteristics

Crystal system: Cubic hexaoctaedrical. 

Color: Red, red-brown, violet, black.

Habit: Rounded grains are typical. Dodecahedrons, trapezohedrons, combined shapes. Massive. Poikiloblastic.

Cleavage: No.

Tenacity: Brittle.

Twinning: Rare

Fracture: Sub conchoidal.

Mohs Hardness: 7 – 7.5

Parting: On {110}.

Streak: White.

Lustre: Vitreous, greasy.

Diaphaneity: Transparent.

Density (g/cm³): 4.3

 

2. Geology and Deposits

Garnets are very common and occur in igneous, metamorphic and sedimentary rocks. They are very resistant and therefore ocurr in sediments derived from the weathering on these rocks.

Almandine is typical, porphyroblastic, in schists, gneisses, granites and pegmatites.

Pyrope occurs in ultramafic igneous rocks (pyroxenites, peridotites). Also in kimberlites and in contact metamorphic rocks.

Spessartite is typical of metamorphic rocks such as gneisses, schists, etc. Also in skarns.

Andradite is rare, from skarns, schists, serpentinites and titaniferous alkaline igneous rocks such as syenites and pyroxenites. A macroscopically black variety, titaniferous (up to 11.5% Ti), typical of alkaline volcanic rocks, is known as “melanite”.

Grossular is a characteristic garnet of gneiss.

Uvarovite is the rarest, found as a secondary mineral in metamorphic contact zones.

 

3. Mineral Associations

Garnets are associated with a very high number of other minerals, whose listing will never cover all situations. It is very commonly associated with quartz, micas (biotite, muscovite, chlorite), staurolite, tourmaline, feldspar (plagioclase, microcline), fluorapatite, beryl and many others.

 

4. Transmitted Light Microscopy

Refraction indices:  n: 1.83 (almandine)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless, more rarely pale pink, pale gray. Due to the very high relief, they simulate brown to cream colors. Never show pleochroism!

Colors are rare: light brown, yellow, reddish, greenish.

In volcanic rocks, melanite garnet has a strong reddish brown color, like the color of dark biotites and brown hornblende.

Relief: Very high (almandine), high (grossular).

Cleavage: None, but there are almost always many fractures.

Habits:

Usually as rounded grains, heavily fractured. Inclusions are very common and can be distributed over the entire grain (sieve texture) or form alignments (spirals) within the garnet grains.

In volcanic rocks, in melanite garnet, are more common cubic euhedral forms, sections with 4, 6 or 8 sides and polygonal sections.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic in most cases.

There are some situations where garnets are anisotropic (see reference cited in the introduction). Situations may occur in which the garnet’s nucleus is isotropic and the edges are anisotropic, usually concentrically zoned.

Extinction: Isotropic.

Elongation sign: No.

Twins: Almandine does not have twins. Other garnets occasionally may be twinned. Anisotropic garnets may show zoning by sectors. This zoning, when well developed, is very similar to a pseudohexagonal twin.

Zoning: Grossular is often zoned. Zonation in anisotropic garnets showing divisions into sectors is common.

CONVERGENT LIGHT

Character: Isotropic, but may be anomalously anisotropic.

2V angle: No.

Alterations: very resistant, it is a common detrital mineral, concentrated in sands.

By hydrothermal processes or low-grade metamorphism it can alter to chlorite, serpentine, hornblende, talc, epidote and iron oxides.

Edges altered to green chlorites, epidote and other minerals are called “keliphytic rims”.

May be confused with: other cubic minerals when isotropic. When anisotropic (which is rare!) it can be confused, in PPL, with other colorless minerals without cleavage and with high relief.

Spinel is similar, but occurs in octahedra and has a strong color.

Perovskite is similar but has even higher relief.

Periclase is very rare and has cleavage.

Vesuvianite is similar to anisotropic garnets with sector zoning.

 

Garnet 01: in PPL, a garnet crystal with typical features: colorless, very high relief, no cleavage, inclusions and fractures.
Garnet 01: the same section as of the previous image, now in XPL. Most garnet are isotropic (black in the 360° rotation of the stage, because they crystallize in the Cubic System).
In PPL, garnets with keliphytic rims, constituted of very small crystals of epidote, chlorite and other minerals.
In the center of the image, rounded garnet grain in PPL with a clear pink color (no pleochroism!). Around it, green amphibole.
Large garnet grain, in XPL, fully altered to chlorite (dark blue and brown colors) and muscovite (colored)
In XPL, large garnet grain (black) with many quartz inclusions (white to gray dots) in mica-schist.
Melanite garnet in PPL in volcanic rock. In this case, the garnet is euhedral, with an 8-sided section. In XPL it is completely black (isotropic).
Garnet in PPL showing zonation with inclusions: inclusions are concentrated in a ring around the grain core.
In PPL, melanite garnet with well-developed zoning in volcanic rock. In XPL, melanite is isotropic (black).
In PPL, garnet with a differentiated texture. Around it, pyroxene and quartz.
In PPL with the maximum magnification objective (40x), in glaucophane-schist, quartz (colorless, low relief), glaucophane (pleochroism between blue and colorless) and garnets (colorless, polygonal, very high relief). Small garnets tend to be more idiomorphic than large garnets. In this case, they tend to have (pseudo)hexagonal outlines.
In PPL, an idiomorphic garnet grain, a situation that is not very common. Garnets are generally xenomorphic.
In PPL, garnet with pressure shadows (“strain shadows”), which are these non-mica areas to the left and right of the garnet grain.
In PPL, garnet grain with alteration to red iron (hydr)oxides on the edges.
In PPL, garnet with alteration to iron (hydr)oxides in its fractures.
In PPL, idiomorphic garnet. The sections are quite variable, depending on the degree of idiomorphism and the section of the grain that is exposed.
In PPL, idiomorphic garnet, tihs time pseudocubic.
In PPL, idiomorphic garnet in a rhombic section.
In PPL, twinned garnets in schist. It is a very rare situation. The twins are evidenced by dark material alignments. The twins are not constant: there are grains with three crystals forming the twin and there are others with complex twins, difficult to interpret. In XPL, the grains are isotropic and show no evidence of the twins. Around it, biotite and quartz.
In PPL, twinned garnet in schist, now in detail.
Garnet 02: Some garnets, such as hydrogrossular and andradite, may appear anisotropic, zoned and sectored. In this crystal the concentric, pseudohexagonal zonation is very well developed.
Garnet 02: The same garnet of the previous image, now in XPL. When rotating the stage, opposite sides extinguish simultaneously .
Garnet 03: Small-grain aggregate of anisotropic skeletal garnets in PPL. The sectorization, combined with zonation, vaguely resembles the zoning (=“fortress texture”) of skutterudite in Reflected Light.
Garnet 03: The same section as of the previous image, now in XPL. The grain contour is difficult to visualize and the low interference colors looks like leucite and nepheline, eventually vesuvianite, which occurs in the same paragenesis.
Garnet 05: isotropic garnet with anisotropic edges of concentric zoning in skarn. Around it, quartz, calcite and clinopyroxenes (diopside?). The next image show this crystals in XPL.
Garnet 05: The same crystals as of the fomer image, now in XPL. Quartz (gray/white), calcite (cream) and diopside(?)(colorful) surround it.

5. Reflected Light Microscopy

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

Sample preparation: Garnet polishing is simple, does not offer any difficulties and is of better quality than the polishing of other silicates that are generally associated with garnets. This great polish – diagnostic! – is due to the relatively high hardness of the garnet and the fact that it has no cleavage. 

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, much lighter than quartz, feldspars in general and especially micas (biotite, muscovite, chlorite).

Pleochroism: No.

Reflectivity: Very low (<<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy is not perceived.

Internal reflections:

Generalized in light red tones, in the case of red garnets. The reflections are quite dark, it’s a good idea to increase the light intensity and/or remove the blue filter to see them better. The shades of red range from almost colorless to a deep red, almost black, depending on the thickness of the garnet at the point considered. In garnets that macroscopically show other colors (yellowish, greenish) the reflections accompany these colors.

In the case of melanite garnet, from volcanic rocks, the reflections are very dark, practically black. The isotropy of the garnet contributes to this black color in XPL.

May be confused with: olivine, which also presents good polishing, but which is usually very fractured and with products of alteration in the fractures. Diagnostic for garnet are rounded shapes, superior polishing, reddish internal reflections and lack of cleavage. Other red transparent minerals may look similar but are much rarer.

Garnet 01: In PPL, in the center of the image, garnet grain with other silicates (quartz, feldspar, etc.) around it. Garnet shows much lighter color than quartz, no cleavage, and excellent polish.
Garnet 01: The same section as in the previous figure, now in XPL. Garnet shows internal reflections that can range from pink to deep red.
Garnet 02: Melanite garnet in fine matrix of a volcanic rock (phonolite), with a grain of sanidine on the right. Polygonal shapes and good polishing are characteristic. Titanite can be very similar and occur in the same paragenesis.
Garnet 02: The same section as in the previous image, now in XPL. Melanite garnet is black due to its color macro, which is black. Sanidine presents, in this image, multicolored reflections.
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