chlorite

A generally green or black secondary mineral, (Mg,Fe,Al)₆(Si,Al)₄O₁₀(OH)₈, often formed by metamorphic alteration of primary dark rock minerals, that appears as a spot of green and resembles mica.

[Latin chlōrītis, a green precious stone, from Greek khlōrītis, from khlōros, green.]

The inorganic group ClO₂ or a salt containing it.

One of a group of layer silicate minerals, usually green in color, characterized by a perfect cleavage parallel to (001). The cleavage flakes are flexible but inelastic, with a luster varying from pearly or vitreous to dull and earthy. The hardness on the cleavage is about 2.5. The specific gravity of chlorate varies between 2.6 and 3.3 as a function of composition.

Chlorite is a common accessory mineral in low- to medium-grade regional metamorphic rocks and is the dominant mineral in chlorite schist. It can form by alteration of ferromagnesian minerals in igneous rocks and is found occasionally in pegmatites and vein deposits. It is a common constituent of altered basic rocks and of alteration zones around metallic ore bodies. Chlorite also can form by diagenetic processes in sedimentary rocks. See also Authigenic minerals; Clay minerals; Diagenesis; Silicate minerals.

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Environment

Commonly a secondary, and often pervasive, mineral like serpentine, but usually affecting localized spots of primary iron, magnesium, aluminum silicates in the rock rather than the entire mass, as is the behavior of serpentines.

Two types form distinct crystals: penninite, though monoclinic, is pseudorhombohedral and forms thick (often triangular) crystals; clinochlore usually grows in broader thinner crystals and is hexagonal in outline. Also fine-grained, in masses, blades, and fibers, or in little rounded knobs and green surfaces defacing nicer crystals (adularia).

Green, black, also brown, redpurple, yellow, and even white. Luster glassy to pearly; hardness 2-2Ɖ; specific gravity 2.6-3.0; cleavage perfect micaceous. Foliaceous and flexible, but not elastic like mica; transparent to opaque.

Chlorite actually is a group name, but it is not practical for the amateur to distinguish between varieties. At best, one can usually assign names only on the basis of appearance, when other tests cannot be given. The chlorites are basic iron, magnesium, aluminum silicates with about 36.1% MgO, 18.4% Al ₂ O ₃ , 32.5% SiO ₂ , and 13.0% H ₂ O. The reddish varieties contain chromium in place of the aluminum, and the reddish brown varieties contain manganese.

Whitens, but fuses only with great difficulty; gives water in the closed tube.

Very flaky. Usually the larger chlorites can easily be distinguished from the micas by their color and the lack of elasticity in the cleavage flakes, and from talc by its greater hardness.

Most commonly as a spot of green alteration of an earlier pyroxene or amphibole in rock; also in chloriterich to almost pure chlorite schists, which may enclose pyrite and/or magnetite crystals. Occasionally crystallized in triangular wedges (penninite) in cavities, as in the alpine crevices or in rocks altered by hot-water solutions. Also in good crystals in Lancaster Co., Pennsylvania. Some of the best crystal (to 2 in.; 5 cm) clinochlore plates were found with magnetite and chondrodite on the surfaces of a serpentinized rock at the old Tilly Foster Mine, Brewster, New York. Also found with talc in Chester Co., Pennsylvania, and in fissures in California serpentine formations with melanite garnet and brown-black, admantine ƈ-in. (2-3 mm) crystals of perovskite (CaTiO ₃ ) near the benitoite locality. The red-purple chromiferous variety (kaemmererite) is well developed in small crystals at Texas, Lancaster Co., Pennsylvania, in some of the chromite mines in California, and in showy Ɖ-in. (1 cm) crystals in Erzincan, East Anatolia, Turkey.

A salt of chlorous acid; disinfectant and bleaching agent.

The chlorite ion

For the mineral type, see chlorite group.

For the neutral chemical compound, see chlorine dioxide.

The chlorite ion is ClO₂⁻. A chlorite (compound) is a compound that contains this group, with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.

Contents 1 Oxidation states 2 Some chlorite compounds 3 Manufacture 4 Usage 5 References

Oxidation states

Chlorine can assume oxidation states of −1, +1, +3, +5, or +7 within the corresponding anions Cl⁻, ClO⁻, ClO₂⁻, ClO₃⁻, or ClO₄⁻, known commonly and respectively as chloride, hypochlorite, chlorite, chlorate, and perchlorate.

oxidation state	−1	+1	+3	+5	+7
anion named	chloride	hypochlorite	chlorite	chlorate	perchlorate
formula	Cl-	ClO−	ClO2−	ClO3−	ClO4−
structure

Some chlorite compounds

• sodium chlorite, NaClO₂
• magnesium chlorite, Mg(ClO₂)₂

Manufacture

The free acid, chlorous acid, HClO₂, is only stable at low concentrations. Since it cannot be concentrated, it is not a commercial product. However, the corresponding sodium salt, sodium chlorite, NaClO₂ is stable and inexpensive enough to be commercially available. The corresponding salts of heavy metals (Ag⁺, Hg⁺, Tl⁺, Pb²⁺, and also Cu²⁺ and NH₄⁺) decompose explosively with heat or shock.

Sodium chlorite is derived indirectly from sodium chlorate, NaClO₃. First, the explosively unstable gas chlorine dioxide, ClO₂ is produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, sodium chloride, sulfur dioxide, or hydrochloric acid).

H₂SO₄(aq) + NaClO₃(s) → NaHSO₄(aq) + HClO₃(aq)
HClO₃(aq) → 2ClO₂(g) + HClO₄(aq) + H₂O(l)

(Other routes for the preparation of chlorine dioxide are available depending on the initial salt).

The chlorine dioxide is then absorbed into an alkaline solution and reduced with hydrogen peroxide, H₂O₂ yielding sodium chlorite (NaClO₂). (Sodium, Na⁺, spectator ions are not shown in the following equations).

2ClO₂(g) + 2OH^- → ClO₂^-(aq) + ClO₃^-(aq) + H₂O(l)
ClO₃^-(aq) + H₂O₂(l) → ClO₂^-(aq) + H₂O(l) + O₂(g)

Usage

The main application of sodium chlorite is the generation of chlorine dioxide for bleaching and stripping of textiles, pulp, and paper. It is also used for disinfection in a few municipal water treatment plants after conversion to chlorine dioxide. An advantage in this application, as compared to the more commonly used chlorine, is that trihalomethanes are not produced from organic contaminants. Sodium chlorite, NaClO₂ also finds application as a component of contact lens cleaning solution under the trade name purite.

Sodium chlorite, like many oxidizers, should be protected from inadvertent contamination by organic materials to avoid the formation of an explosive mixture.

References

• Chemistry of the Elements, N.N. Greenwood and A. Earnshaw, Pergamon Press, 1984.
• Kirk-Othmer Concise Encyclopedia of Chemistry, Martin Grayson, Editor, John Wiley & Sons, Inc., 1985

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