Arsenopyrite
Iron arsenic sulfide (FeAsS)
A silver-white iron arsenic sulfide and the most common arsenic mineral, known for striking sparks and a garlic smell when struck.
- Mohs hardness
- 5.5-6
- Color
- silver-white to steel-gray, tarnishing dull
- Type
- mineral
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Overview
Arsenopyrite is the most abundant arsenic-bearing mineral and the principal ore of arsenic. Its chemical formula is FeAsS, an iron arsenic sulfide that crystallizes in the monoclinic system, often forming well-shaped prismatic and wedge-shaped crystals.
Its old miners' name, mispickel, dates back centuries. Freshly broken surfaces are bright silver-white, but the mineral tarnishes to a dull gray with exposure.
Arsenopyrite is important to prospectors because it commonly carries gold, and its presence can signal nearby precious-metal mineralization.
Formation & geology
Arsenopyrite forms mainly in medium- to high-temperature hydrothermal veins and is one of the earliest sulfides to crystallize. It also occurs in pegmatites, contact metamorphic deposits (skarns), and in association with tin and tungsten ores.
It is frequently found alongside gold, pyrite, chalcopyrite, galena, sphalerite, cassiterite, and wolframite. Because gold often occurs as microscopic inclusions within arsenopyrite, the mineral is an important pathfinder and ore for gold mining.
Notable localities include Freiberg in Germany, Cornwall in England, Sweden, Portugal, and numerous gold districts worldwide.
How to identify it
Look for a silver-white to steel-gray metallic mineral, often as elongated prismatic or wedge-shaped crystals with striated faces. It is hard (5.5-6), so it will scratch glass, unlike softer sulfides.
The streak is dark gray to black. A diagnostic test: when struck with a hammer or steel, arsenopyrite gives off sparks and a distinctive garlic (arsenic) odor.
Look-alikes include marcasite and pyrite, which are more brassy-yellow, and skutterudite. The combination of silver-white color, high hardness, crystal habit, and the garlic smell when struck is highly diagnostic.
Uses & significance
Arsenopyrite is the chief ore of arsenic, used historically in pesticides, wood preservatives, and certain alloys and semiconductors. Today most arsenic is recovered as a byproduct of processing other metal ores.
It is also significant as a gold-bearing ore, since refractory gold deposits often host their gold within arsenopyrite grains. Mining and processing require careful handling because of arsenic toxicity.
For collectors, sharp crystals are prized specimens. It has essentially no jewelry or metaphysical use and should be stored carefully and handled with washed hands due to its arsenic content.
Frequently asked questions
Is arsenopyrite poisonous?
Arsenopyrite contains arsenic, which is toxic. Solid specimens are safe to handle briefly, but avoid inhaling dust, never ingest it, and wash your hands after handling; store it away from food and acids.
How can I tell arsenopyrite from pyrite?
Arsenopyrite is silver-white rather than brassy-yellow, is somewhat harder, and gives off a garlic smell and sparks when struck.
Does arsenopyrite contain gold?
Often yes. Gold frequently occurs as microscopic inclusions in arsenopyrite, making it an important gold ore and exploration indicator.
Why does arsenopyrite smell like garlic?
When struck or heated, it releases arsenic compounds that produce a characteristic garlic-like odor, a classic field test.
What is mispickel?
Mispickel is the traditional miners' name for arsenopyrite.
Arsenopyrite guides
In-depth guides for identifying, valuing, and understanding Arsenopyrite.
Arsenopyrite identified by the community
Real specimens identified with Rock Identifier.
