Minerals containing Iron

Copperas or melanterite, FeSO₄.7H₂O, occurs in nature as the result of the oxidation of pyrites or marcasite. It yields greenish, monoclinic crystals, of hardness 2, and density when pure 1.832. A rhombic variety of the heptahydrate is known as tauriscite. Heat decomposes the heptahydrates, yielding at 200° to 250° C. the monohydrate, FeSO₄.H₂O, found in nature as ferropalladite, and at high temperatures, ferric oxide. Copperas is isomorphous with Boothite, CuSO₄.7H₂O, and with pisanite, (Fe, Cu)SO₄.7H₂O, which latter occurs in California as blue, transparent crystals. Copperas has also been found as a bluish green crust on limonite in Carinthia. Its density is 1.8 to 1.95. The .isomorphism of these minerals is evident from the following data: -

	a: b: с	β
Melanterite	1.1828: 1: 1.5427	75° 44'
Pisanite	1.1670: 1: 1.5195	75° 30'
Boothite	1.1622: 1: 1.5000	74° 24'

All these minerals lose 6 molecules of water at 200° to 250° C.

Salvadorite, FeSO₄.2CuSO₄.21H₂O, appears to be a dimorphous variety of pisanite. A ferric sulphate, Fe₂(SO₄)₃.9H₂O, occurring in hexagonal prisms in Coquimbo is known as Coquimbite. It is white to yellowish brown in colour. Janosite is a greenish yellow efflorescence occurring in Hungary, and possessed of the same composition. It crystallises in rhombic plates. The decahydrate, Fe₂(SO₄)₃.10H₂O, occurs as monoclinic crystals in quenstedtite. Ferronatrite, Na₃Fe(SO₄)₃.3H₂O, occurs as a greenish-white mineral in Chili.

Halotrichite or ferrous alum, FeSO₄.Al₂(SO₄)₃.24H₂O, occurs in several localities, including Persia, where it is used by the natives for making ink. In Baluchistan it occurs as a white inflorescence on decomposed slate, and is used for dyeing, its local name being khaghal. It has recently been found in Elba in fibrous masses, white or faintly blue in colour, and possessed of a silky lustre. Density 1.90; hardness 2.5. Analysis shows a slight deficit of water due to partial dehydration in dry air.

Other sulphates are copiapite, 3Fe₂O₃.8SO₃.27H₂O; amarantite, Fe₂O₃.2SO₃.7H₂O; fibroferrite, Fe₂O₃.2SO₃.10H₂O; castanite, Fe₂O₃.2SO₃.8H₂O; botryogen, FeO.MgO.Fe₂(SO₄)₃.18H₂O; roemerite, FeSO₄.Fe₂(SO₄)₃12H₂O; planoferrite, Fe₂O₃.SO₃.15H₂O; paposite, 2Fe₂O₃.3SO₃.10H₂O. Iron associated with tellurium occurs as durdenite, Fe₂O₃.3TeO₂.4H₂O, in Honduras, and as emmonsite in Colorado.

Rhabdite is a crystalline phosphide of iron approximating in composition to (Ni, Fe)₃P. It is found in meteorites in the form of minute tetragonal prisms. Density 6.3 to 6.8.

Vivianite, Fe₃(PO₄)₂.8H₂O, occurs as monoclinic crystals, isomorphous with erythrite, Co₃(AsO₄)₂.8H₂O, and perhaps also with annabergite, Ni₃(AsO₄)₂.8H₂O. Its crystallographic elements are: -

a: b: c = 0.7489: 1: 0.7017. β = 75° 34'.

Some specimens obtained from Cornwall are beautifully crystallised, possessing a pale, bluish green tint. Others are nodular, deep blue in colour, pulverulent, and sometimes enclose crystals of dark brown colour, which exhibit a reddish hue by reflected light. Both the green and the brown specimens yield the same results upon analysis, so that their difference in colour is most probably due to a difference in the degree of oxidation of a small portion of the iron. The crystals offer a very perfect cleavage parallel to the clino-pinacoid. The pure mineral is probably colourless, the tints usually observed being due to some of the iron becoming oxidised. The density ranges from 2.6 to 2.7. Some good crystals have yielded the value of 2.587.

The mineral was discovered by Vivian, in Cornwall, whence its name.

An earthy variety fojmd in peat mosses is known as blue iron earth.

In paravivianite, from Russia, part of the iron is replaced by manganese and magnesium, thus (Fe, Mn, Mg)₃(PO₄)₂.8H₂O. Kertschenite, (Fe, Mn, Mg)O.Fe₂O₃.P₂O₅.7H₂O, likewise found in Russia, occurs as very dark green crystals.

Tamanite, the same mineral as anapaite, (Ca, Fe)₃(PO₄)₂.4H₂O, occurs in pale greenish crystalline masses.

An anhydrous mixed phosphate of manganese, calcium, and divalent iron occurs as Graftonite, (Fe, Mn, Ca)₃(PO₄)₂, which occurs as salmon-coloured, monoclinic crystals, the crystallographic elements of which are

a: b: c = 0.886: 1: 0.582. β = 66°.

Triphyllite, Li(Fe, Mn)PO₄, occurs as light green, rhombic crystals, which become dark through oxidation.

Ludlamite is a green hydrated form of ferrous phosphate found in Cornwall and elsewhere. On heating it disintegrates into bluish green plates, in contradistinction to vivianite which turns white and exfoliates under similar treatment.

Ferric phosphate occurs in nature as phospho-siderite, 4FePO₄.7H₂O, and strengite, FePO₄.2H₂O. Possibly these two minerals are the same.

A basic ferric phosphate, known as Dufrenite, Fe₂(HO)₃.(PO₄), is found as rhombic crystals, dark green in colour, and named after Dufrenoy, the French mineralogist. Hardness 3.5 to 4; density 3.2 to 3.4. A ferric phosphate, Fe₂O₃.P₂O₅.6H₂O or FePO₄.3H₂O, occurs in monoclinic needles as Koninckite in Belgium.

Beraunite is FePO₄.2Fe₂(PO₄)(HO)₃.4H₂O, and occurs in reddish brown monoclinic crystals. Ghildrenite is a hydrated phosphate of iron and aluminium with a little manganese. It crystallises in yellow or brown rhombic prisms. When more manganese is present the mineral is called eophosphorite.

Lolingite, FeAs2, occurs in Saxony and Norway in rhombic crystals. Hardness 5 to 5.5; density 6.8 to 8.7. Greyish in colour, it gives a greyish black streak. When heated in an open tube a white sublimate of arsenious oxide or " white arsenic " is obtained; in the absence of air metallic arsenic is volatilised to a dark sublimate.

Leucopyrite, FeAs.FeAs₂, resembles the previous mineral in appearance, hardness, and density.

Berthierite, FeS.Sb₂S₃, occurs in elongated rhombic prisms of density 4 to 4.3; hardness 2 to 3. It has a steely appearance. Heated in air, oxides of sulphur and antimony are evolved. It is readily soluble in hydrochloric acid, evolving hydrogen sulphide.

Scorodite, FeAsO₄.2H₂O, occurs as pale green or brown rhombic crystals of density 3.1 to 3.3; hardness 3.5 to 4. It owes its name (Greek skorodon, garlic) to the fact that, when heated, it emits the characteristic odour of arsenic. It occurs in Cornwall, and in a readily oxidisable form as a deposit from certain geysers in Yellowstone Park, U.S.A. A hydrated arsenate of iron and aluminium is known as Liskeardite.

A basic arsenate, 2FeAsO₄.Fe(HO)₃.5H₂O, is known as iron sinter, or pharmaco-siderite in consequence of the poisonous character of arsenic (Greek pharmakon poison). It crystallises in small, green cubic crystals, for which reason it is also known as cube ore.

A hydrated arsenate of iron and calcium from Yukon is termed Yukonite. Its composition approximates to (Ca₃,Fe₂)As₂O₈.2Fe(HO)₃.5H₂O. It is brownish black, resinous, and amorphous. Density c. 2.65.

Ferrous antimonate, 2FeO.Sb₂O₅ or FeSbO₃, is found in the cinnabar gravels of Tripuhy, and is termed Tripuhyte. It is dull, greenish yellow in colour, possessing a canary-yellow streak. When heated, antimony oxide vaporises, leaving ferric oxide behind. Density 5.82. Derbylite is ferrous antimonate associated with titanate, 2FeSbO₃. 5FeTiO₃. It crystallises in the rhombic system, its elements being

a: b: c = 0.96612: 1: 0.55025.

It is black in colour, with a resinous lustre.

An ortho-silicate of iron, termed fayalite or iron olivine, Fe₂SiO₄, was first found on Fayal Island in the Azores. It had crystallised in tabular rhombs of Fe₂SiO₄, very similar to the ferrous silicate present in certain slags. The axial ratios are

a: b: c = 0.46000: 1: 0.58112.

It is quite possible that it was simply a lump of slag from a ship's ballast. Fayalite has also been found at Rockport.

Olivine, Fe₂SiO₄ + xMg₂SiO₄, is a variable mixture of ferrous and magnesium ortho-silicates, olive green in colour, and of density about 3.3 to 3.5. It is a common constituent of rocks.

Anthophyllite, or magnesium iron metasilicate, (Mg, Fe)S₂O₃, crystallises in the rhombic system. A pure iron anthophyllite, FeSiO₃, has been found at Rockport, but is rare. It is white to light brown in colour.

Sodium ferric metasilicate is found as acmite or aegirite, Na₂Fe₂(SiO₃)₄. Its crystals are blackish green in colour, and possess the following crystallographic elements: -

a: b: c = 1.1044: 1: 0,6043. β = 73° 27'.

Hydrated ferric metasilicates are found as Miillerite, Fe₂O₃.3SiO₂.2H₂O, and nontronite, Fe₂O₃.3SiO₂.5H₂O.

Astrolite, (Al, Fe)₂Fe(Na, K)₂(SiO₃)₅.H₂O, a metasilicate of aluminium, iron, and alkali metals, occurs in Saxony as small spherical balls, with radially fibrous structure.

Glauconite, the green constituent of many rocks, if pure, would be represented by the formula KFe(SiO₃)₂.H₂O, that is, potassium ferric metasilicate.

Cronstedtite occurs in Cornwall, associated with vivianite. It is also met with in association with Hisingerite in acicular needles in cavities in iron pyrites. Its usual structure is fibrous and radiating, and it consists essentially of ferrous silicate and hydrated ferric oxide.

Hisingerite likewise occurs in Cornwall, as dark brown, amorphous, reniform masses. Its hardness is 2.75, and density 1.74. It yields a conchoidal fracture and leaves a rust-brown streak. Its composition approximates to Fe₂O₃.2SiO₂.2H₂O.

Other silicates are pinguite, 2Fe₂O₃.6SiO₂.3H₂O + xH₂O; hoeferite, 2Fe₂O₃.4SiO₂.7H₂O.

Numerous other silicates are known in which iron is one of the constituent metals. As a source of iron they are all useless for commercial purposes, since the percentage of the metal is low.

Wolframite, (Fe, Mn)WO₄, is a black mineral associated with cassiterite in Cornwall and elsewhere. It yields monoclinic crystals which may be distinguished by their lustre and perfect single cleavage. Hardness 5.5; density 7.3.

Ferritungstite, Fe₂O₃.WO₃.6H₂O, an alteration product of Wolframite, is a pale ochre, which under the microscope is seen to consist of hexagonal plates.

Ferrous chloride is present inrinneite, FeCl₂.3KCl.NaCl, which occurs in large, lenticular masses in beds of rock salt at Wolkramshausen in the Harz, and as a kieserite-rinneite rock in Hanover. It crystallises in the hexagonal system, and when a hot, saturated solution is allowed to cool, potassium chloride separates out.

Douglasite, FeCl₂.2KCl.2H₂O, occurs in the Stassfurt deposits, associated with carnallite. Density 2.16.

Ferric chloride has been found as a brown incrustation on lava, particularly in the neighbourhood of Vesuvius. It is then known as molysite, FeCl₃. In combination with alkali chlorides, ferric chloride is found as kremersite, KCl.NH₄Cl.FeCl₃.H₂O, and Erythrosiderite, 2KCl.FeCl₃.H₂O, both minerals being found in minute quantities in the neighbourhood of Vesuvius.

Lagonite is an ochre-coloured incrustation consisting of iron borate, Fe₂O₃.3B₂O₃.3H₂O. It occurs as an incrustation by lagoons in Tuscany. A natural nitride of iron has been found in the lava of Etna, as the mineral siderazote.

For the sake of easy reference, the foregoing minerals are given in the accompanying tables, together with their chemical compositions and more important physical characteristics.