Chlorides, Natural

a class of minerals; salts of hydrochloric acid, HCl. Two groups of natural chlorides are distinguished according to composition, properties, and conditions of formation. The first group, which includes 28 minerals, comprises the soluble hydrous and anhydrous chlorides of Na, K, NH₄, Mg, Ca, Al, Mn, and Fe.

The principal minerals of this group are halite, NaCl, sylvite, KCl, sal ammoniac, NH₄Cl, bischofite, MgCl₂ · 6H₂O, carnallite, KMgCl₃ · 6H₂O, tachyhydrite, CaMgCl₄ · 12H₂O, and rinneite, NaK₃FeCl₆. They contain 20 to 70 percent chlorine. The structural unit of the crystal comprises a dense isometric packing of Cl atoms. The atoms of the metals are located in the octahedral vacancies. The chemical bonds are mainly ionic.

Chlorides of the first group crystallize in the isometric or trigonal system. They are usually colorless, and their hardness on Mohs’ scale ranges from 1 to 2, and their density from 1,600 to 3,200 kg/m³. These minerals are hygroscopic and are readily soluble in water and partially soluble in alcohol. They are salty or bitter to the taste. They occur in the form of granular and compact masses and as veins and nodules in sedimentary layers. Crystal concretions, crusts, and incrustations form in lake sediments, solonchaks, and the products of volcanic and fumarolic activity. Many minerals of this group are widely used in the chemical and food industries and in agriculture. (See alsoPOTASSIUM SALTS and ROCK SALT.)

The second group, which includes 49 minerals, comprises the insoluble chlorides of Cu, Pb, Ag, Hg, As, Sb, and Bi, often with the additional anions O^– and OH^– and sometimes with [NO₃]^–, Fe^–, and [SO₄]²⁺. The principal minerals are nantokite, CuCl, atacamite, Cu₂Cl(OH)₃, connellite, Cu₁₉Cl(OH)₃₂SO₄ · 4H₂O, laurionite, PbCl(OH), matlockite, PbFCl, mendipite, Pb₃Cl₂O₂, chlorargyrite, AgCl, calomel, Hg₂Cl₂, diaboleite, PbCuCl₂(OH)₄, and boleite, Pb₃Cu₃AgCl₇(OH)₆. The minerals of this group contain 6 to 35 percent chlorine.

Chlorides of the second group crystallize mainly in the orthorhombic or tetragonal system. Layered structures, with a relatively high covalence of the chemical bonds, are common. The minerals are colorless, dark blue, green, or yellow. They occur in the form of crusts, crystal aggregates, incrustrations, or earthy masses. The hardness on Mohs’ scale varies to 4, and the density ranges from 3,700 to 8,300 kg/m³. These chlorides, which are insoluble in water, form in the oxidation zones of ore deposits under arid climatic conditions; they also form upon the action of salt water on ore minerals and industrial slag during volcanic activity. Those contained in oxidized ores are used as raw materials in the production of copper, lead, and silver.