SULPHUR

Sulphur – S₈ – is a relatively common native element, characteristic of various geological environments. The mineral is written “sulphur”; the commodity is written “sulfur”. Sulphur is an important mineral, as sulfur has many industrial applications.

Sulphur is classified in the Sulphur Group and is a polymorph of rosickyite (monoclinic S, very rare) and β-sulfur. It may contain some Se and Te, giving rise to two varieties (with Se, with Te). It is generally easy to recognize due to its yellow color and very unctuous feel. By striking or rubbing sulfur, H2S (hydrogen sulfide gas) is generated, with the typical rotten egg smell.

Sulphur crystals are common, especially well-developed in volcanic sublimate occurrences. More than 50 forms have been recorded, reaching sizes of up to 20 cm. Bipyramidal crystals are very common, and they can be tabular or sphenoidal.

Caution! It burns easily in air at just 113ºC with a blue flame, forming a strong-smelling oxide that, once inhaled, reacts with the moisture in the trachea and lungs, forming sulfuric acid!

1. Characteristics

Crystal system: Orthorhombic bipyramidal.

Color: Yellow, can show greenish, reddish, orange tones. Gray, brown. Black if mixed with organic matter.

Habit: Massive, stalactitic, reniform, powdery, spongy, crusty.
Crystals are common (see above).

Cleavage: {001} imperfect, {110} imperfect, {111} imperfect

Tenacity: Brittle, it could be something sectile.

Twinning: Rare, by {101}, {011} and {110}.

Fracture: Conchoidal to irregular.

Mohs Hardness: 1.5 – 2.5

Parting: in {111}.

Streak: White, colorless.

Lustre: Resinous to greasy, adamantine on the surfaces.

Diaphaneity: Transparent.

Density (g/cm³): 2.07

2. Geology and Deposits

Sulphur occurs in volcanic fumaroles, where it forms by sublimation from volcanic gases.

In sedimentary rocks (evaporites), sulphur is common, forming voluminous deposits through the bacterial reduction of sulfates. Large volumes of sulphur form on top of salt domes, also through the bacterial decomposition of calcium sulfate. It is also deposited in hot springs.

It can be found in hydrothermal vents, associated with sulfides. It forms in coal mine fires from pyrite. It occurs in cavities, massive aggregates, or as pore filling in the oxidation zone of sulfide deposits, neoformed on galena, sphalerite, and pyrite.

3. Mineral Associations

Depending on the type of occurrence, the assemblage of associated minerals varies considerably:

– Salt dome tops: halite, gypsum, and anhydrite.

– Volcanic sublimates: realgar, cinnabar, volcanic glass, and many others.

– Hydrothermal veins: calcite, barite, stibnite, galena, pyrite, and other sulfides.

– Cementation zones of sulfide deposits: pyrite, sphalerite, galena, anglesite, “stibioconite,” and others.

– Sedimentary rocks: calcite, aragonite, dolomite, sulfates, halides, and celestine.

4. Transmitted Light Microscopy

Refraction indices: n_α: 1.958 n_β: 2.037 n_γ: 2.245

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Pale yellow or yellowish-gray color, with distinct pleochroism ranging from light yellow to dark yellow.

Relief: Very high, extreme.

Cleavage: distinct to bad, {001} and {110}. The partition directions parallel to the four faces {111} of the bipyramid may be well developed, better than the cleavages.

Habits: Short prismatic grains, thick tabular or skeletal shapes, “hopper”. May form aggregates of very fine grains and coloform or crust-like masses.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.287, very high, with diffuse, creamy, white colors that are difficult to classify, reminiscent of the interference colors of carbonates.

Extinction: symmetrical with respect to the parting directions and parallel or symmetrical to the cleavages in the principal sections.

Elongation sign: ES(+) on the longer diagonal in diamond-shaped sections.

Twins: Rarely, simple contact twinning can occur.

Zoning: No.

CONVERGENT LIGHT

Character: B(+), usually with many isochromatic colors, which makes it difficult to obtain interference figures and determine their character.

2V angle: 69%

Alterations: sulphur alters to gypsum and anhydrite.

May be confused with: few other minerals. The extreme relief and pleochroism in yellow tones are diagnostic.

5. Reflected Light Microscopy

Sample preparation: Due to its low hardness, it is extremely difficult to polish. It is softer than gypsum. Polishing grooves usually remain, and the sulfur has a negative relief relative to the surrounding minerals (except in relation to gypsum).

It must be thoroughly dried; otherwise, the hydrogen reacts with the sulfur, forming H2S and deteriorating the polished section. Warm water dissolves native sulphur. Heat, even moderate heat, produces cracks – therefore, it is important not to hold it in your hand, avoid sunlight, and observe it only briefly under a microscope if using a halogen lamp instead of an LED.

Granular aggregates are very difficult to impregnate, especially since sulfur burns at 113ºC, so the resins cannot be heated too much.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark white gray.

Pleochroism: Clear, very evident in intergranular contacts, sometimes it can be observed in individual grains.
Low-quality polishing masks this pleochroism.

Reflectivity: 12% (very low)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy masked by internal reflections.

With the nicols uncrossed at 5–10º, anisotropy can sometimes be recognized when the grains are not too small.

Internal reflections: Intense, abundant, white to yellow. Very small crystals tend to have colorless reflections, while large crystals generally show yellowish reflections.

Due to the effect of double refraction, internal reflections can be seen in pairs, an observation that is not always easy and requires experience.

May be confused with: Few other minerals.

Diagnostic features include low hardness, yellow color, absence of cleavage, intense internal reflections, and a smell when struck. It is easy to recognize.

Cerussite has some similarity, it is somewhat lighter in color, and its polishing quality is much higher.

Sphalerite has a much higher reflectivity, despite also being low.

General Characteristics:

Grain shape: In significant occurrences, the crystals are relatively large, provided there is no clay contamination. When native sulphur originates from the alteration of sulfides, the grains are very small, rounded to polygonal. It can form crusts, be earthy, fill fractures, or form rhythmic (banded) crusts. In cementation zones, it forms large crystals in cavities.

Cleavage is not observed in the polished section.

Polishing grooves will always be present, making it difficult to visualize pleochroism and anisotropy.

Sulphur cement can occur between fragments of volcanic glass in volcanic deposits.

Inclusions of volcanic ash can occur in volcanic deposits.