Hematite

Class: Oxides & hydroxides
Group: Hematite

An accessory mineral in felsic igneous rocks, a late-stage sublimate in volcanic rocks, and in high-temperature hydrothermal veins. A product of contact metamorphism and in metamorphosed banded iron formations. A common cement in sedimentary rocks and a major constituent in oolitic iron formations (Anthony et al., 2001—2005).

Hematite — specimen 0171, photo © NMNHS

Original description: unknown [prehistoric].

Type locality: unknown [prehistoric].

Type material: unknown.

Etymology: from the Ancient Greek αἱματίτης (haimatites), bloodlike, in allusion to vivid red colour of the powder.

Distribution: widespread. Algeria: Nador; Brazil: Bahia, Itabira and Brumado mine; Minas Gerais, Mesa Redonda and Congonhas do Campo; England: Cumbria, Cleator Moor; Italy: Elba, Rio Marina; Norway: Kragerø and Hiassen; Romania: Ocna de Fier; South Africa: North Cape, Kuruman district; Switzerland: St Gotthard, Uri; Binntal, Valais; and many other places; USA: Arizona, La Paz Co., near Quartzsite; Utah, Juab Co., Thomas Range (Anthony et al., 2001—2005).

Chemistry

Fe2O3

Essential elements: oxygen (O), iron (Fe).

Crystal data

Crystallography: trigonal — hexagonal scalenohedral. Crystal habit: crystals complex rhombohedral, pseudocubic, prismatic, rarely scalenohedral, to 13 cm, with triangular striations on {0001} and {1011}; thin tabular, to micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or stalactitic masses, columnar; earthy, granular, oolitic, concretionary. Twinning: penetration twins on {0001} or with {1010} as composition plane; also lamellar on {1011} (Anthony et al., 2001—2005).

Physical properties

Cleavage: parting on {0001} and {1011} due to twinning (Anthony et al., 2001—2005). Fracture: uneven to subconchoidal (Anthony et al., 2001—2005). Tenacity: brittle; elastic in thin laminae (Anthony et al., 2001—2005). Hardness: 6 (Lazzarelli, 2012). Density: 5.26—5.30 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: steel-grey, may have iridescent tarnish, dull to bright red; white to grey-white, with a bluish tint, in reflected light with deep blood-red internal reflections (Anthony et al., 2001—2005). Diaphaneity: оpaque; translucent on thin edges (Anthony et al., 2001—2005). Lustre: metallic or submetallic to dull (Anthony et al., 2001—2005). Refractive index: 2.69—3.22 — anisotropic [uniaxial (-)] (Lazzarelli, 2012). Birefringence: 0.28. Dispersion: very strong (Anthony et al., 2001—2005). Pleochroism: weak; O = brownish red; E = yellowish red (Anthony et al., 2001—2005).

Material from ‘Repository’

5 specimens: 0031 — 2.78 ct, Tanzania; 0405 — 2.55 ct, Tanzania; 0407 — 3.19 ct, Tanzania; 0440 — 2.37 ct, Tanzania; 0171 — 3.48 ct, Tanzania.

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