How to identify Sulfur-coated Basalt

To correctly identify Sulfur-coated Basalt (Basalt with native sulfur), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Basalt is dull to sub-vitreous. Native sulfur has a resinous to greasy luster, sometimes adamantine in well-formed crystals.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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