TOURMALINE

Tourmaline – NaFe3Al6(Si6O18)(BO3)3(OH)3(OH) (Schorl) – is a relatively common cyclosilicate that can occur in many types of rock. It is a mineral with very high prices in the market of collectible minerals and as gemstone.

“Tourmaline” is actually not a mineral, but just a generic term applied to members of the Tourmaline Group, which is made up of more than 30 different minerals with a huge compositional variety. The most common tourmaline is schorl, which is black color in hand specimen.

Due to the large variations in color and some variations in habit, around two dozen varieties of tourmaline are recognized.

Under the microscope, the differentiation of members of the Tourmaline Group is difficult, almost impossible, requiring the use of other analytical techniques.

1. Characteristics

The characteristics below refer to schorlite.

Crystal system: Trigonal, piramidal ditrigonal.

Color: Bluish black to black, sometimes brownish black, rarely greenish black.

Habit: Trigonal prisms with convex faces, can be granular to acicular.

Cleavage: {11-20} poor, {10-11} poor.

Tenacity: Brittle.

Twinning: Rare.

Fracture: Irregular, conchoidal.

Mohs Hardness: 7                                 

Parting: No.                             

Streak: Grayish white to bluish.

Lustre: Vitreous, greasy.

Diaphaneity: Transparent.

Density (g/cm³): 3.18 – 3.22

 

2. Geology and Deposits

Tourmaline is typical of granitic pegmatites and high temperature hydrothermal veins, being accessory in granites, granodiorites and related felsic rocks.

In metamorphic rocks, it is common in schists, gneisses, quartzites and phyllites. It also occurs in dolomites and metasomatic limestones (skarns) in contact metamorphic zones, as well as in some cornubianites.

Sometimes it occurs in greisen and also in albitites related to carbonatites.

It is found in sediments (placers) and sedimentary rocks such as sandstones (graywackes).

 

3. Mineral Associations

In granitic pegmatites, it is associated with the typical minerals of this paragenesis: quartz (rock crystal and smoky crystal), albite (cleavelandite), potassic feldspar (microcline), mica (muscovite, lepidolite), beryl (aquamarine), fluorite, topaz and others.

In metamorphic rocks, paragenesis changes and the number of associated minerals increases.

Dravite, another very common tourmaline, is associated with quartz, feldspar (albite), micas (muscovite, phlogopite), carbonates (calcite, magnesite), corundum, anatase and pyrite.

 

4. Transmitted Light Microscopy

Refraction indices:  no: 1.660 – 1.672     ne: 1.635 – 1.650

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless or intensely colored in yellow, orange, blue, green, red, black and brown, with pleochroism.

Longitudinal sections show strong pleochroism; basal sections have very weak pleochroism or no pleochroism.

Colors are often unevenly distributed, with spots and zones of different shades, with or without a defined pattern (edges/center).

When it occurs in very dark colors, it can simulate an opaque mineral.

Relief: Moderate to high.

Cleavage: {110} and {101} weak, very poorly developed, not normally visible in thin section.

Habits: Long prismatic with fractures perpendicular to the elongation (diagnostic!).

Basal sections can be rounded, triangular or hexagonal.

May form radial aggregates of microcrystals (“suns”). It can form completely xenomorphic (anhedral) grains, which make it difficult to recognize.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Moderate to high birefringence, maximum 0.025, (or 0.035 or 0.040), with 2nd order colors: orange, red, blue.

The intense colors of tourmalines often mask the interference colors.

Extinction: Parallel in the longitudinal sections.

Elongation sign: ES(-), often difficult to recognize due to the intense own colors of the tourmaline and the high interference colors.

Twins: Rare.

Zoning: Often zoned; zones tend to triangular in basal sections, but can be very irregular, in various spotted colors.

CONVERGENT LIGHT

Character: U(-), can be biaxial under stress, see 2V angle on the side. Use basal sections to see figure!

2V angle: May have a small anomalous 2V angle of up to 10°.

Alterations: Never alters. Tourmaline and quartz are two minerals that are never found with alteration.

May be confused with: strong pleochroism and uniaxial character are diagnostic. Very typical is the uneven distribution of colors, with irregular patches of different hues distributed over the grains.

Black tourmalines with deep colors can be confused with opaque minerals.

Tourmalines with strong colors and in the basal sections, without pleochroism, are more difficult to identify.

Very small tourmalines require more attention for their identification.

Arfvedsonite may be similar in that it also shows deep, dark colors.

Dumortierite, which can be blue or pink, can be quite similar.

Sillimanite, when fibrous and pink, may be similar but has ES(+).

Longitudinal section of tourmaline in PPL with its typical characteristics: long prismatic habit, strong and zoned colors, strong to weak pleochroism, fractures perpendicular to the elongation, absence of cleavage and high relief. Above the longitudinal section, there is a basal (roundish) section.
Tourmaline 01: Tourmaline (dravite) in longitudinal section, with a prismatic habit and typical fractures perpendicular to the elongation. This is the first color of pleochroism, the second one is shown in the next image. PPL.
Tourmaline 01: The same crystal as of the previous image, now rotated 90 degrees to show the second color of pleochroism. PPL.
Tourmaline 01: The same crystal of the two previous images, now in XPL. The colors are so strong that they mask the interference colors. In the vertical position the crystal will be extinct.
In PPL, basal section of tourmaline. The “official” form of the basal section is a triangle with convex faces. However, they are not easy to find and, therefore, they are not a feature that can be used routinely to identify tourmaline.
In PPL, basal section of tourmaline. The “official” form of the basal section is a triangle with convex faces. However, they are not easy to find and, therefore, they are not a feature that can be used routinely to identify tourmaline. Such very dark-colored tourmalines may be confused with opaque minerals!
Basal pseudohexagonal section of tourmaline. This time its color is blue. Pleochroism of basal sections is weak to absent! PPL.
In PPL, very dark tourmalines, easily confused with opaque minerals! Using the condenser, its identification as transparent minerals is easier.
In PPL, blue-colored tourmalines with weak pleochroism. The interference figures help to identify tourmalines.
In PPL, brownish tourmalines. The two crystals show almost perfectly the two colors of pleochroism. Zoning is evident, the fratures perpendicular to the "z" axis are typical for tourmaline.
In PPL, tourmalines with a very strong orange color.
Tourmaline 02: In PPL, orange-colored tourmalines, with zoning and a weak pleochroism. Cleavage is absent, relief is high.
Tourmaline 02: The same section as of the previous image, now in XPL. These tourmalines show strong, colorful interference colors. Quartz is gray/white.
In PPL, tourmalines with a capillary habit (high relief) in quartz (low relief). The tourmalines form long and very thin, colorless prisms that form tufts with a radial disposition or they occur randomly dispersed in the quartz crystals. Only the larger tourmaline crystals show any color, and, even then, pleochroism is difficult to spot. Diagnostic for tourmaline are parallel extinction and negative elongation sign in XPL.

5. Reflected Light Microscopy

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

Sample preparation: tourmaline acquires easily a great polish. Polishing is faster and is of better quality than the polish of quartz, feldspars and calcite, for example.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like quartz and feldspars. The same color as the darker pleochroism color of calcite.

Pleochroism: No.

Reflectivity: Very low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy is not observed.

Internal reflections: Generalized, with the same color as the tourmaline macroscopically and with a hue that depends on the thickness of the tourmaline at the point considered: the thinner the tourmaline, the lighter the reflections; when thicker, darker, reaching black regardless of the macro color.

May be confused with: the great polishing, the habit when idiomorphic, the reflection colors and the parallel extinction are very diagnostic.

General Characteristics: 

Polishing scratches were not observed, even with poor polishing.

Polishing pits do not occur.

Extinction is paralell in the longitudinal sections.

Fractures perpendicular to the elongation are very typical for tourmaline.

Tourmaline 01: Diagonally in the image, prismatic tourmaline crystal in PPL. Above it, quartz. Below, calcite. In black, polishing imperfections (holes). In hand specimen, this tourmaline is almost black.
Tourmaline 01: The same sections as of the previous image, now in XPL. Internal reflections are greenish yellow.
In XPL, schorlite (black tourmaline) and quartz. In this case, the tourmaline is thick and practically black. Only at the finer tips, there are greenish hues.
In XPL, blue tourmaline, showing blue colors in various shades, which can evolve to bluish black.
In XPL, red tourmaline (rubellite) on the left and quartz on the right. The rubellite shows soft pinkish tones that evolve into deep, almost black tones. When many fractures and imperfections are present, the pink color becomes more evident.
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