Arsenopyrite Identification Guide

What Arsenopyrite Looks Like

Arsenopyrite (FeAsS) is a silver-white to steel-gray metallic mineral, often with a faint yellowish or pinkish tarnish. It has a bright metallic luster and is opaque. Crystals are common and diagnostic: typically prismatic, wedge-shaped, or rhombic (pseudo-orthorhombic), often striated, and frequently forming twins or cross-shaped/star groups. It also occurs as granular and massive aggregates in veins.

Step-by-Step Field ID Checklist

1. Note the metallic silver-white color — brighter and more silvery than brassy pyrite.
2. Examine crystal shape — look for elongated, wedge-shaped or rhomb-shaped prisms with lengthwise striations; this habit strongly suggests arsenopyrite.
3. Check the streak on unglazed porcelain — dark gray to black.
4. Do the hardness test — moderately hard (5.5–6); a knife barely scratches it.
5. Perform the smell/spark test (carefully) — strike or hammer a fragment: arsenopyrite sparks and gives off a distinct garlic (arsenic) odor. This is the single most diagnostic field test.

Key Diagnostic Tests

• Hardness: 5.5–6, harder than most sulfides except pyrite/marcasite.
• Streak: Dark gray to black.
• Specific gravity: High, ~5.9–6.2 — it feels noticeably heavy.
• Cleavage: Distinct in one direction but often hard to see; fracture uneven.
• Garlic odor: When struck with a hammer or heated, it emits a strong garlic smell from volatilized arsenic — diagnostic. Handle with care: arsenopyrite contains arsenic. Wash hands, do not inhale dust or fumes, and never lick or heat it indoors.
• Color/tarnish: Fresh surfaces are silver-white; tarnishes to gray or with iridescent/pinkish hues.

Common Look-Alikes and How to Tell Them Apart

• Pyrite: Pyrite is distinctly brass-yellow (not silver-white), commonly forms cubes/pyritohedra, and does not smell of garlic when struck. Arsenopyrite's silvery color and garlic odor separate them.
• Marcasite: Paler, more tin-white, often with a greenish tarnish; tabular or cockscomb forms; less arsenic smell. Crystal habit and weaker garlic odor distinguish it.
• Skutterudite / cobaltite: Can be silvery and arsenical; cobaltite tends toward silver-white with a reddish tinge and cubic habit. These are rarer; chemistry/locality help.
• Galena: Galena is softer (2.5), has perfect cubic cleavage and a lead-gray color with bright cleavage planes; much softer and no garlic smell.

Where Arsenopyrite Is Found

Arsenopyrite is the most common arsenic mineral, found in medium- to high-temperature hydrothermal veins, often with gold, tin, tungsten, and other sulfides; in pegmatites and contact-metamorphic deposits. Notable localities include Freiberg (Germany), Cornwall (England), Panasqueira (Portugal), and many gold-mining districts worldwide, where it is an important indicator of gold mineralization.