Sulfur
Sulfur (S, native element)
A bright yellow native element mineral that forms around volcanic vents and hot springs and burns with a blue flame.
- Mohs hardness
- 1.5-2.5
- Color
- bright yellow to yellow-brown or greenish yellow
- Type
- mineral
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Overview
Sulfur (also spelled sulphur) is one of the few elements that occurs in nature in its native, uncombined form. Its vivid lemon-to-canary yellow color makes it one of the most instantly recognizable minerals.
It crystallizes in the orthorhombic system, forming blocky, dipyramidal crystals as well as powdery crusts and earthy masses. Sulfur is soft, brittle, and a poor conductor; it has a very low melting point and ignites readily, burning with a blue flame and a pungent, choking sulfur-dioxide smell — the 'brimstone' of antiquity.
Specimens are popular with collectors, but they are fragile and sensitive to heat, even cracking from the warmth of a hand.
Formation & geology
Native sulfur forms primarily in volcanic and hydrothermal settings. Around fumaroles, volcanic vents, and hot springs, sulfur-bearing gases (hydrogen sulfide and sulfur dioxide) react and deposit bright yellow sulfur crusts and crystals directly from the vapor (sublimation).
Large bedded deposits form in sedimentary evaporite and salt-dome environments, where bacteria reduce gypsum/anhydrite sulfate to sulfur. These cap-rock deposits, as along the US Gulf Coast and in Poland and Sicily, were historically the main commercial source.
Fine crystals come from Sicily, and active sulfur mining still occurs at volcanoes such as Indonesia's Kawah Ijen.
How to identify it
Sulfur is identified by its bright yellow color, low hardness (1.5-2.5), resinous-to-greasy luster, and pale yellow-to-white streak. It is brittle and crumbly, and crystals are commonly translucent and blocky.
A cautious sniff of a freshly rubbed surface, or its tendency to crack from gentle heat, helps confirm it. It does not conduct heat well, so it feels warm quickly. (It will ignite if heated strongly — handle carefully.)
Look-alikes: Orpiment is also yellow but more orange and has a different luster; it is an arsenic sulfide and toxic. Yellow varieties of calcite or barite are harder and lack the resinous look and sulfur smell.
Uses & significance
Sulfur is a foundational industrial chemical, overwhelmingly used to make sulfuric acid, one of the most produced chemicals in the world, essential for fertilizers, batteries, and countless processes.
It is also used in vulcanizing rubber, manufacturing matches, gunpowder, fungicides, and pharmaceuticals, and as a soil amendment in agriculture. Today most commercial sulfur is recovered from oil and gas processing rather than mined.
Historically known as brimstone, sulfur has religious and medicinal associations. Bright crystal specimens are collected for their color, though they must be protected from heat and handling.
Frequently asked questions
Why does sulfur smell like rotten eggs?
Pure sulfur is nearly odorless, but it is associated with hydrogen sulfide gas, which smells of rotten eggs; burning sulfur produces sharp, choking sulfur dioxide.
Is sulfur safe to handle?
Solid native sulfur is generally safe to touch, but it is flammable, can crack from body heat, and its dust and fumes can irritate the eyes and lungs.
What is brimstone?
Brimstone is the old name for sulfur, derived from 'burning stone,' referring to its ability to ignite and burn with a blue flame.
Why do sulfur crystals crack so easily?
Sulfur is a poor heat conductor with a very low melting point, so the warmth of a hand can heat the surface faster than the interior, building stress that cracks the crystal.
Sulfur guides
In-depth guides for identifying, valuing, and understanding Sulfur.
Sulfur identified by the community
Real specimens identified with Rock Identifier.
