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Figure 1. A fine specimen of the iron oxide, hematite, displaying a botryoidal to reniform surface (the terms respectively refer to grape-like or kidney-like shapes). The 1-euro coin is 23 mm in diameter. Sample from Stenelux of Amsterdam, Netherlands.
"Rock of the Month #164, posted for February 2015" ---
Hematite
is a grey to reddish-black iron oxide,
rhombohedral
α-Fe2O3,
with a distinct reddish streak.
This metallic grey oxide has a Mohs hardness of 5-6 and a specific gravity
of circa 5.26, quite similar to magnetite
(Fe3O4).
It may have a tabular, platy habit, with aligned shiny faces,
the variety known as specularite.
The reniform variety (Fig. 1) is
known, in northern England especially, as "kidney ore".
Unlike magnetite, it is not appreciably magnetic.
On broken faces, a radiating crystal structure
can be seen to underlie the individual botryoidal
surface features.
Hematite is but one of some 215 mineral species
recorded from the geologically diverse Bou Azzer district of Morocco
(Favreau et al., 2007).
In reflected light, hematite is a light bluish-grey, pale against magnetite
and ilmenite. It is anisotropic and often shows red internal reflections.
Pale margins against gangue are often blood-red in transmitted light (Fig. 2).
It may form "martite" pseudomorphs after magnetite.
The habit is tabular, and the elongate crystals are often aligned
along the foliation in hematite-bearing schists.
Figure 2.
A sample of hematite schist from Tigrai, northern Ethiopia.
The sample is from the Mato Bula Au-Cu-Zn-Pb trend at the Adyabo project of
East Africa Metals Inc.
Note the occurrence of
tabular, platy hematite crystals within a foliated matrix composed mainly of quartz and mica.
The translucent blood-red colour of tiny flakes and the thin margins of
larger crystals is a useful diagnostic feature: the oxyhydroxide goethite
is a darker blue-grey in reflected light, and commonly has a lighter,
orange to red hue in transmitted light
Two photomicrographs, nominal magnification 200X, long-axis field-of-view
0.45 mm.
In order to highlight the hematite crystal habit, the plane-polarized
transmitted light is augmented with a lesser amount of reflected light.
Hematite is most notably found in giant
Precambrian sediment-hosted iron deposits of hematite-rich banded iron formation (BIF)
around the world.
Some examples include
Krivoy Rog (Ukraine), the Hamersley basin
(Western Australia), and Carajas and the Quadrilatero Ferrifero (Brazil;
see Dalstra and Guedes, 2004), as well
as the deposits of the Fort Gouraud area, Mauritania,
such as the F'Derik orebody
(Gross and Strangway, 1961), and other examples in Canada (e.g., Ontario
and Quebec), the
U.S.A. (e.g., Michigan and Minnesota:
Han, 1982). India, Liberia and elsewhere.
It is possible that some
hematite enrichment ores may be of
diagenetic origins (Findlay, 1994).
Minute spherules of hematite, 120-200 nm (nanometres, i.e., 0.12-0.2 microns)
in diameter have been detected in the lowest member of the
Hamersley Group.
This banded iron formation is dated near the
Archean-Proterozoic boundary, circa 2500 Ma.
These nanospheres
may have formed by dehydration of colloidal Fe hydroxide particles
(Ahn and Buseck, 1990).
Iron oxides formed by decomposition of
pre-existing sulphides may take up
percent levels of chalcophile metals from those
minerals, as an invisible
dowry of Cu and other elements
(Rose and Bianchi-Mosquera, 1993).
Hematite in BIF may be derived by oxidation of pyrite,
or from
primary
magnetite-banded oxide BIF (Symons, 1967).
Hematite occurs also in veins, as in the
Snowdon Volcanic Group,
the remains of an Ordovician caldera in north Wales
(Colman and Appleby, 1991).
The Manto Verde Cu deposit in northern Chile
displays specularite-rich breccias
(Vila et al., 1996).
Hematite is also abundant in many IOCG (iron oxide copper-gold)
deposits, a good example being Prominent Hill
in South Australia (Belperio et al., 2007).
In IOCG deposits, there is often
replacement of magnetite by "martite" (hematite pseudomorphs),
associated with other minerals such as actinolite
(Skirrow, 2010).
References
Ahn,JH and Buseck,PR (1990) Hematite nanospheres of possible colloidal origin from a
Precambrian banded iron formation. Science 250, 111-113, 05 October.
Belperio,A, Flint,R and Freeman,H (2007) Prominent Hill:
a hematite-dominated, iron oxide copper-gold system.
Econ.Geol. 102, 1499-1510.
Colman,TB and Appleby,A-K (1991) Volcanogenic quartz-magnetite-hematite veins, Snowdon, North
Wales. Mineral.Mag. 55, 257-262.
Dalstra,H and Guedes,S (2004)
Giant hydrothermal hematite deposits with Mg-Fe metasomatism:
a comparison of the Carajas, Hamersley, and other iron ores.
Econ.Geol. 99, 1793-1800.
Favreau,G, Dietrich,JE, Meisser,N, Brugger,J, Haddouch,LA and
Maacha,L (2007) Bou Azzer, Morocco.
Mineral.Record 38, 345-407.
Findlay,D (1994) Diagenetic boudinage, an analogue model for the control on
hematite enrichment iron ores of the Hamersley Iron Province of
Western Australia, and a comparison with Krivoi Rog of Ukraine,
and Nimba Range, Liberia. Ore Geology Reviews 9, 311-324.
Gross,WH and Strangway,DW (1961)
Remanent magnetism and the origin of hard hematites in
Precambrian banded iron formation. Econ.Geol. 56, 1345-1362.
Han,TM (1982) Iron formations of Precambrian age: hematite-magnetite
relationships in some Proterozoic iron deposits - a microscopic
observation. In `Ore Genesis, the State of the Art' (Amstutz,GC, El Goresy,A,
Frenzel,G, Kluth,C, Moh,G, Wauschkuhn,A and Zimmermann,RA
editors), Springer-Verlag, 804pp., 451-459.
Rose,AW and Bianchi-Mosquera,GC (1993)
Adsorption of Cu, Pb, Zn, Co, Ni, and Ag on goethite and
hematite: a control on metal mobilization from red beds into
stratiform copper deposits.
Econ.Geol. 88, 1226-1236.
Skirrow,RG (2010) "Hematite-group" IOCG±U ore systems:
tectonic settings, hydrothermal characteristics,
and Cu-Au and U mineralizing processes. In `Exploring for Iron Oxide
Copper-Gold Deposits: Canada and Global Analogues'
(Corriveau,L and Mumin,H editors),
GAC Short Course Notes 20, 185pp., 39-58.
Symons,DTA (1967) Paleomagnetic evidence on the origin of the Marquette and Steep
Rock hard hematite and goethite deposits. Can.J.Earth Sci. 4, 1-20.
Vila,T, Lindsay,N and Zamora,R (1996)
Geology of the Manto Verde copper deposit, northern Chile:
a specularite-rich, hydrothermal-tectonic breccia related to the Atacama
fault zone.
In `Andean Copper Deposits: New Discoveries, Mineralization, Styles and
Metallogeny' (Camus,F, Sillitoe,RM and Petersen,R editors),
SEG Spec.Publ. 5, 157-170.
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Related samples are BIF from Karnataka state in south India (ROM 98) and Western Australia (ROM 88)