Magnetite
Fe₃O₄ · Cubic · also called lodestone, black sand
Magnetite is the black, strongly magnetic iron oxide that makes up most of the 'black sand' left in the bottom of a gold pan. The magnet test is definitive: among common minerals, only magnetite responds strongly to a household magnet.
Properties
- Mohs hardness
- 5.5–6.5
- Streak
- black
- Lustre
- metallic, submetallic
- Colour
- Iron black throughout, with no colour variation.
- Cleavage
- No cleavage; sometimes shows octahedral parting.
- Fracture
- Conchoidal to uneven
- Specific gravity
- 5.1–5.2 (water = 1)
- Magnetism
- Strongly magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm magnetite
- Strongly magnetic — a fridge magnet picks it up. Nothing else common behaves this way.
- Black streak, which separates it instantly from hematite.
- Often occurs as sharp octahedra, and as the black grains left in a gold pan.
What magnetite gets confused with
Each of these is settled by one test. Run that test first.
Streak. Hematite always leaves red-brown; magnetite leaves black. Hematite is at most weakly magnetic.
Chromite has a brown streak and is only weakly magnetic at best.
Where it is found
Igneous and metamorphic rocks worldwide; concentrated in beach and stream sands because it is heavy and durable.
Is it worth anything?
Essentially no cash value except for well-formed octahedral crystals. Naturally magnetised pieces (lodestone) are sold as curiosities.