Sulfur-coated Basalt

Igneous rock with mineral coating

Basalt with native sulfur

Also known as: Basalt with native sulfur, Sulfurous Basalt

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Description

Sulfur-coated basalt is a dark, fine-grained igneous rock (basalt) that has been partially or completely covered by a layer of native sulfur. The sulfur typically appears as bright yellow, often crystalline, coatings, crusts, or infillings within vesicles and fractures of the basalt. The contrast between the dark basalt and the vibrant yellow sulfur makes it visually distinctive. The basalt itself is a mafic extrusive igneous rock, rich in iron and magnesium, and typically fine-grained due to rapid cooling.

How to Identify

Color: Basalt is typically dark gray to black. The sulfur coating is bright yellow, lemon-yellow, or greenish-yellow. The intensity of the yellow can vary.
Luster: Basalt is dull to sub-vitreous. Native sulfur has a resinous to greasy luster, sometimes adamantine in well-formed crystals.
Texture: Basalt is typically aphanitic (fine-grained) to porphyritic, sometimes vesicular. The sulfur coating can be powdery, massive, or crystalline, often forming botryoidal or stalactitic growths within vesicles.
Crystal Form: Basalt minerals (plagioclase, pyroxene, olivine) are usually too small to be seen without magnification. Native sulfur often forms distinct orthorhombic crystals (dipyramidal, tabular), granular masses, or encrustations.
Cleavage: Basalt minerals exhibit cleavage, but the rock itself does not have a distinct cleavage plane. Native sulfur has imperfect cleavage on {001}, {110}, and {111}.
Geological Environment: Active volcanic regions, fumaroles, solfataras, hot springs, and areas with recent basaltic volcanism and associated hydrothermal alteration.

Key Facts

Hardness: Basalt: 5-6 on Mohs scale (for constituent minerals). Native Sulfur: 1.5-2.5 on Mohs scale.
Specific Gravity: Basalt: 2.8-3.0 g/cm³. Native Sulfur: 2.07 g/cm³.
Crystal System: Basalt: Polycrystalline (minerals are typically monoclinic or triclinic). Native Sulfur: Orthorhombic.
Color: Dark gray to black (basalt); bright yellow, lemon-yellow, greenish-yellow (sulfur).
Luster: Dull to sub-vitreous (basalt); resinous to greasy, sometimes adamantine (sulfur).
Transparency: Opaque (basalt); transparent to translucent (sulfur).
Fracture: Conchoidal to uneven (basalt); conchoidal to uneven (sulfur).
Cleavage: None for the rock (basalt); imperfect on {001}, {110}, {111} for native sulfur.
Composition: Basalt: Primarily plagioclase feldspar (labradorite), pyroxene (augite), and often olivine. Native Sulfur: Elemental sulfur (S).

Quick Check

Color: Dark gray to black basalt with bright yellow sulfur coating.
Luster: Dull to sub-vitreous for basalt; resinous to greasy for sulfur.
Streak: White for basalt (if powdered); yellow for native sulfur.

Physical Characteristics

Crystal Habit: Basalt: Massive, aphanitic, porphyritic, vesicular. Native Sulfur: Massive, granular, earthy, stalactitic, botryoidal, or well-formed dipyramidal or tabular crystals.
Cleavage Type: Basalt: Not applicable as a rock. Native Sulfur: Imperfect.
Fracture Type: Conchoidal to uneven for both basalt and sulfur.
Tenacity: Basalt: Brittle. Native Sulfur: Brittle.
Luster Type: Basalt: Dull to sub-vitreous. Native Sulfur: Resinous to greasy.

Formation

Sulfur-coated basalt forms when basaltic lava flows or volcanic rocks are exposed to sulfur-rich volcanic gases or hydrothermal fluids. Native sulfur (S) precipitates and crystallizes on the surface and within vesicles of the basalt. This process is common in active volcanic and geothermal areas where hydrogen sulfide (H2S) gas is oxidized to elemental sulfur, or where sulfur dioxide (SO2) reacts with H2S.

Usage

Primarily a geological curiosity and a specimen for collectors. It can indicate past or present volcanic/hydrothermal activity. Native sulfur itself has industrial uses (e.g., sulfuric acid production, fertilizers, pharmaceuticals), but sulfur-coated basalt is not typically mined for its sulfur content due to the low concentration and difficulty of extraction.

Age Distribution

Variable, depending on the age of the basaltic eruption and subsequent hydrothermal activity. Can range from recent (Holocene) to ancient (Precambrian) where sulfur deposition occurred later.

Where to Find

Volcanoes of Sicily, Italy

Mount Etna and Stromboli are well-known for active fumaroles and sulfur deposition on basaltic rocks.

Kilauea Volcano, Hawaii, USA

Active volcanic vents and fumaroles on basaltic lava flows often exhibit sulfur deposition.

Kamchatka Peninsula, Russia

Numerous active volcanoes and geothermal fields with extensive sulfur mineralization on volcanic rocks, including basalt.

Indonesia

Volcanic islands like Java and Sumatra have many active volcanoes where sulfur precipitates on basaltic and andesitic rocks.

Iceland

Geothermal areas and active rift zones with basaltic volcanism can host sulfur deposits.

Finding Tips

Look for Volcanic Activity

Focus on areas with active or recently active volcanoes, especially near fumaroles, solfataras, and hot springs where sulfurous gases are emitted.

Observe Color and Luster

The bright yellow color and resinous luster of native sulfur are highly distinctive against the dark, dull basalt.

Check for Vesicles and Fractures

Sulfur often precipitates within the gas bubbles (vesicles) and cracks of the basalt, so examine these features closely.

Smell for Sulfur

In active areas, a faint smell of sulfur (rotten eggs, H2S) might be present, indicating the source of the sulfur.

Safety Precautions

Always be aware of the hazards in volcanic and geothermal areas, including toxic gases (H2S, SO2), hot ground, and unstable terrain. Wear appropriate protective gear, including a respirator if sulfurous gases are strong. Avoid direct contact with sulfur if possible, as it can be irritating.

Similar Rocks

Pyrite-coated Basalt

Basalt with pyrite (FeS2)

Also known as: Fool's Gold Basalt

Gypsum-coated Basalt

Basalt with gypsum (CaSO4·2H2O)

Also known as: Selenite Basalt

Opal-coated Basalt

Basalt with opal (SiO2·nH2O)

Also known as: Hyalite Basalt

Scientific Classification

Mineral Class: Basalt: Igneous rock. Native Sulfur: Native Element.
Group: Basalt: Volcanic rock. Native Sulfur: Sulfur Group.
Crystal System: Basalt: Polycrystalline. Native Sulfur: Orthorhombic.
Chemical Formula: Basalt: Complex silicate (e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene, (Ca,Na)(Al,Si)4O8 for plagioclase). Native Sulfur: S.
Composition: Basalt: Predominantly plagioclase feldspar, pyroxene, and often olivine, with minor magnetite and ilmenite. Native Sulfur: Pure elemental sulfur.

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