Cassiterite Identification Guide
Identify cassiterite, the chief tin ore, by its high density, adamantine luster, and distinctive crystals, and separate it from look-alikes.
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What Cassiterite Looks Like
Cassiterite is tin oxide (SnO2), the principal ore of tin. It is notably heavy and often shows a brilliant, almost sub-metallic luster.
- Color: usually dark brown to black; also red-brown, yellow, or (rarely) near-colorless/honey.
- Luster: adamantine to sub-metallic, often greasy on fractures.
- Transparency: translucent to opaque (gem-quality crystals can be transparent).
- Habit: stubby tetragonal prisms with pyramidal ends; characteristic "elbow" (knee-shaped) twins; also massive, granular, and as water-worn "stream tin" pebbles.
Step-by-Step Field-ID Checklist
- Heft it: cassiterite feels surprisingly heavy for its size (density ~6.8–7.1) — the standout clue.
- Check the streak: scrape on unglazed porcelain — white to pale brown/gray, NOT black like most heavy dark minerals.
- Test hardness: about 6–7; scratches glass and steel.
- Look for twinning: distinctive elbow/knee-shaped twin crystals are diagnostic.
- Note the luster: brilliant adamantine sheen on crystal faces despite the dark color.
Key Diagnostic Tests
- Mohs hardness: 6–7.
- Streak: white to pale brownish or grayish — a crucial separator from black metallic minerals.
- Density: very high, ~6.8–7.1 g/cm³ (heavier than most common minerals).
- Cleavage: poor; fracture conchoidal to uneven.
- Magnetism: not magnetic.
- Tin test: a fragment in dilute HCl with zinc develops a dull tin coating (classic assay).
Common Look-Alikes
- Black tourmaline (schorl): much lighter (density ~3.1) and forms long striated prisms; cassiterite is far denser with elbow twins.
- Rutile: similar adamantine luster and tetragonal habit, but lower density (~4.2) and a pale brown streak; rutile is often redder.
- Wolframite/columbite: also heavy and dark, but they give dark/brown-black streaks and differ in habit.
- Magnetite: heavy and black, but magnetic with a black streak — cassiterite is non-magnetic with a pale streak.
- Sphalerite: has resinous luster, perfect cleavage, and lower density.
Where Cassiterite Is Found
Cassiterite occurs in high-temperature hydrothermal veins and greisens associated with granites, in pegmatites, and concentrated in alluvial "stream tin" placers. Major sources include Bolivia, Malaysia, Indonesia, China, Cornwall (England), and Nigeria.
Frequently asked questions
How can you tell if something is cassiterite?
Cassiterite is unusually heavy (density ~7), has a brilliant adamantine luster, a hardness of 6–7, and — critically — a white-to-pale-brown streak rather than the black streak of most dark heavy minerals. Elbow-shaped twin crystals confirm it.
What is cassiterite used for?
It is the world's primary ore of tin, mined from veins, pegmatites, and alluvial placer deposits.
How do you tell cassiterite from black tourmaline?
Cassiterite is far denser (about 7 vs 3.1 for tourmaline) and forms stubby twinned crystals, while black tourmaline forms long striated prisms and feels comparatively light.
Does cassiterite have a colored streak?
Its streak is white to pale brownish or grayish, which is diagnostic because it separates cassiterite from black, metallic-streaking minerals like magnetite and wolframite.