Magnetite

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Iron(II,III) oxide (Fe₃O₄); the most strongly magnetic naturally occurring mineral and one of the two principal iron ore minerals alongside hematite. Black to dark grey, metallic luster, specific gravity ~5.18 g/cm³, Mohs hardness 5.5–6. Theoretical iron content 72.4 wt% Fe, somewhat higher than hematite (69.9 wt%), but magnetite ores are often intimately intergrown with silica and other gangue minerals, reducing effective grade. Magnetite can be reduced to iron in a bloomery furnace by CO, proceeding through wüstite (FeO) as an intermediate: Fe₃O₄ + CO → 3FeO + CO₂; FeO + CO → Fe + CO₂. Has the inverse spinel crystal structure (space group Fd3̄m, no. 227) and is strongly ferrimagnetic — the most magnetic of all naturally occurring minerals, giving it the alternative name ‘lodestone’ in its naturally magnetized form. Stable at atmospheric pressure from ambient temperature to approximately 1597°C, above which it decomposes. [Sources: Klein, C. & Dutrow, B. (2007), ‘Manual of Mineral Science’, 23rd ed., Wiley, pp. 333–336; Kubaschewski, O. & Alcock, C.B. (1979), ‘Metallurgical Thermodynamics’, 5th ed., Pergamon, pp. 267–271 — for reduction sequence and thermodynamics.]

Common forms

• Lump ore (direct feed to bloomery)
• Sand (beach placer) — used in some Japanese tatara furnace tradition
• Pelletized concentrate (industrial)

Common sources

• Magmatic segregation deposits (e.g., Kiruna, Sweden — historically important European source)
• Banded iron formations (often interlayered with hematite)
• Beach placer deposits (black sand magnetite)
• Skarn deposits

Composition

Fe₃O₄: one FeO unit and one Fe₂O₃ unit per formula unit. Contains ~72.4 wt% Fe in the stoichiometric compound (calculated: 3×55.845 / (3×55.845 + 4×16.00) = 167.535 / 231.535 = 0.7236). Fe²⁺ occupies octahedral sites; Fe³⁺ occupies both tetrahedral and octahedral sites in the inverse spinel structure. Natural ores typically 45–65 wt% Fe due to silica and other gangue.

Hazards

• Fine dust respiratory irritant; similar considerations as hematite (siderosis risk from chronic inhalation)
• No acute toxicity in bulk form

Properties

• color: Black; streak black — distinguishes from hematite (red-brown streak)
• magnetic: Strongly ferrimagnetic — attracted to hand magnet; diagnostic field test distinguishing it from hematite
• reducibility: Fe₃O₄ → FeO (wüstite) → Fe; reduction from magnetite to wüstite occurs above ~570°C; wüstite to iron above ~700°C [CIT-04, pp. 267–271]
• mohs_hardness: 5.5–6
• crystal_system: Isometric (cubic), inverse spinel
• specific_gravity: ~5.18 g/cm³
• stability_window: Stable at atmospheric pressure from ambient temperature to approximately 1597°C, above which it decomposes to hematite + liquid iron oxide. Reduced to wüstite (FeO) by CO above ~570°C in a reducing atmosphere. [CIT-04]
• crystal_structure: Inverse spinel (cubic), space group Fd3̄m (no. 227). Strongly ferrimagnetic.
• iron_content_stoichiometric: ~72.4 wt% Fe (stoichiometric) [CIT-MAG-01]

Purity grades

• High-grade concentrate: 65–72% Fe (after magnetic separation)
• Run-of-mine ore: 45–60% Fe

Claims

• Magnetite (Fe₃O₄) is reduced to wüstite (FeO) by CO above approximately 570°C: Fe₃O₄ + CO → 3FeO + CO₂. (confidence 0.95; sources: CIT-04)
  • Standard metallurgical thermodynamics; consistent with the Ellingham diagram and the Direct Reduction of Iron Oxides concept node. The ~570°C threshold is the same value used in the Wüstite (FeO) and Bloomery Iron Smelting nodes.
• Magnetite contains approximately 72.4 wt% Fe in the stoichiometric compound. (confidence 0.95; sources: CIT-04)
  • Calculable from atomic masses: 3×55.845 / (3×55.845 + 4×16.00) = 167.535 / 231.535 = 0.7236. Standard figure confirmed in any inorganic chemistry or mineralogy reference.
• Magnetite has the inverse spinel crystal structure (space group Fd3̄m, no. 227) and is strongly ferrimagnetic. (confidence 0.97; sources: CIT-MAG-01)
  • Well-established mineralogy; consistent across all standard references. Space group Fd3̄m (no. 227) is the canonical assignment for inverse spinel. The ferrimagnetism arises from antiparallel alignment of Fe²⁺ and Fe³⁺ magnetic moments on different sublattice sites.

Needs verification

Upper stability limit ~1597°C for magnetite at atmospheric pressure. (non-blocking)

Figure given from standard Fe–O phase diagram knowledge (consistent with Kubaschewski & Alcock 1979) but no direct page citation for the exact decomposition temperature has been verified from a primary source in this node. Non-blocking because the value is standard and appears in all major Fe–O phase diagram references.

Connections

Incoming

• Extracted from ← Wüstite (FeO) — Wüstite (FeO) is derived from magnetite (Fe₃O₄) by reduction with CO above ~570°C: Fe₃O₄ + CO → 3FeO + CO₂. This is step (2) of the direct reduction sequence (Fe₂O₃ → Fe₃O₄ → FeO → Fe). In the bloomery furnace context this occurs in the mid-temperature zone of the shaft. Wüstite is then further reduced to metallic iron at ~700°C+. Source: Kubaschewski & Alcock (1979), pp. 267–271. Edge re-pointed from draft duplicate (da4cbd5d) to canonical committed Magnetite node during Cycle 3 repair.
• Requires input ← Bloomery Iron Smelting — Magnetite (Fe3O4) is an alternative iron ore feedstock to hematite; used in bloomery smelting where locally available, e.g., beach placer sands in Japanese tatara tradition. Reduced in stages: Fe3O4 → FeO → Fe.
• Requires input ← Blast Furnace Ironmaking — Magnetite (Fe3O4) is the secondary principal iron ore feedstock; requires magnetic concentration from ore body. Processed through same reduction sequence as hematite.

Sources

• CIT-MAG-01 · Klein, C.; Dutrow, B. (2007) Manual of Mineral Science. 23rd ed., Wiley, pp. 333–336. — Primary mineralogical reference for magnetite: crystal structure (inverse spinel), hardness, specific gravity, streak, magnetic properties, and ore occurrences.
• CIT-04 · Kubaschewski, O.; Alcock, C.B. (1979) Metallurgical Thermodynamics. 5th ed., Pergamon, pp. 267–271. — Authoritative thermodynamic reference for the iron oxide reduction sequence: Fe₃O₄ + CO → 3FeO + CO₂ (above ~570°C); FeO + CO → Fe + CO₂ (above ~700°C). Also provides the ~1597°C upper stability limit context for the Fe–O phase diagram.