Hematite

Hematite
Brazilian trigonal hematite crystal
General
CategoryOxide minerals
Formula
(repeating unit)
iron(III) oxide, Fe2O3, α-Fe2O3[1]
IMA symbolHem[2]
Strunz classification4.CB.05
Dana classification4.3.1.2
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H–M symbol: (3 2/m)
Space groupR3c (no. 167)
Unit cella = 5.038(2) Å;
c = 13.772(12) Å; Z = 6
Identification
ColorMetallic grey, dull to bright "rust-red" in earthy, compact, fine-grained material, steel-grey to black in crystals and massively crystalline ores
Crystal habitTabular to thick crystals; micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or stalactitic masses, columnar; earthy, granular, oolitic
TwinningPenetration and lamellar
CleavageNone, may show partings on {0001} and {1011}
FractureUneven to subconchoidal
TenacityBrittle
Mohs scale hardness5.5–6.5
LusterMetallic to splendent
StreakBright red to dark red
DiaphaneityOpaque
Specific gravity5.26
Density5.3
Optical propertiesUniaxial (−)
Refractive indexnω = 3.150–3.220, nε = 2.870–2.940
Birefringenceδ = 0.280
PleochroismO: brownish red; E: yellowish red
References[3][4][5]

Hematite (/ˈhməˌtt, ˈhɛmə-/), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils.[6] Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of Fe
2
O
3
. It has the same crystal structure as corundum (Al
2
O
3
) and ilmenite (FeTiO
3
). With this it forms a complete solid solution at temperatures above 950 °C (1,740 °F).

Hematite occurs naturally in black to steel or silver-gray, brown to reddish-brown, or red colors. It is mined as an important ore mineral of iron. It is electrically conductive.[7] Hematite varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While these forms vary, they all have a rust-red streak. Hematite is not only harder than pure iron, but also much more brittle. Maghemite is a polymorph of hematite (γ-Fe
2
O
3
) with the same chemical formula, but with a spinel structure like magnetite.

Large deposits of hematite are found in banded iron formations. Gray hematite is typically found in places that have still, standing water, or mineral hot springs, such as those in Yellowstone National Park in North America. The mineral may precipitate in the water and collect in layers at the bottom of the lake, spring, or other standing water. Hematite can also occur in the absence of water, usually as the result of volcanic activity.

Clay-sized hematite crystals also may occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, which is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.

  1. ^ Dunlop, David J.; Özdemir, Özden (2001). Rock Magnetism: Fundamentals and Frontiers. Cambridge: Cambridge University Press. p. 73. ISBN 9780521000987.
  2. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  3. ^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Hematite" (PDF). Handbook of Mineralogy. Vol. III. Chantilly, VA: Mineralogical Society of America. ISBN 978-0962209727. Retrieved December 22, 2018.
  4. ^ "Hematite Mineral Data". WebMineral.com. Retrieved December 22, 2018.
  5. ^ "Hematite". Mindat.org. Retrieved December 22, 2018.
  6. ^ Cornell, Rochelle M.; Schwertmann, Udo (1996). The Iron Oxides. Germany: Wiley. pp. 4, 26. ISBN 9783527285761. LCCN 96031931. Retrieved December 22, 2018.
  7. ^ Morgenthau, Mengo L. (1923). Minerals and Cut Stones: Reference Book Containing Condensed and Simplified Descriptions from Standard Works on Mineralogy. p. 23.