Morphological characteristics of few minerals

Some substances with analogous formulae and similarity in thy relative sizes of cations and anions, show closely related crystal structure. This feature is isomorphism and the concerned substances are said to be isomorphous. Since there is similarity in the internal crystal structure, the substances crystallize with similar external forms. With the same type of cleavage. Spinel, garnet, amphibole and carbonate mineral groups are known to display isomorphism. The property is explained in the cases of calcite (trigonal crystallization) and aragonite (orthorhombic crystallization), as given here under. (a) Calcite (CaCO3). Siderite (FeCO3). Rhodochrosite (MnCO3), magnesite (MgCO3) and smithsonite (ZnCO3) have a narrow range of interfacial angle between the faces (1011), and (1101), the amount being 72. 12, (b)The interfacial angles between the faces (110) and (110) in aragonite (CaCO3), cerussite (PbCO3). Witherite (BaCO3) and Strontianite (SrCO3) are more or less similar, the range being 62.30. to 63.48 Studies suggest that elements with cations larger than that of calcium crystallise with orthorhombic symmetry and those with cations smaller than calcium, crystallise with trigonal symmetry. In case a substance with a particular chemical composition crystallizes with different structures then polymorphism is exhibits, The development structures is attributable to different environments of crystallization, including temperature and pressure, Some examples are given hereunder.

·       Diamond and graphite are both constituted of carbon. While diamond crystallizes with cubic symmetry under high temperature and pressure, graphite with hexagonal structure does not need the environmental conditions of diamond for its formation.

·       The polymorphs of silica include quartz, tridymite, cristobalite coesite and stishovite. The stability fields of the polymorphs, are as follows;

Ø  Quartz (hexagonal-trigonal symmetry) - Stable at low temperatures

Ø    Tridymite (orthorhombic symmetry) - Stable between 867 and 1470  

Ø    Cristobalite (cubic symmetry) - Stable above 1470  

Ø  Coesite (monoclinic symmetry) - High pressure environment

Ø  Stishovite (tetragonal symmetry) - High pressure environment.

Marcasite (FeS2) change is over to pyrite (FeS2) but the reverse does not happen such one- sided change is known as monotropy. Enantiopy, in which under the influence of a definite temperature and pressure a polymorph changes to the original, is displayed by quartz and tridymite as shown below;

·      867  under one atmosphere

·       Quartz, Tridymite

Recent studies bear out the graphite can be changed to diamond under high pressure and temperature. The conversion of diamond to graphite has been known for long. So. an enantiotropy relationship exists between graphite and diamond. In case a mineral is replaced by another material without any change in external form, the phenomenon of pseudo morphism is exhibited. Some examples are given below;

·       Copper (Cu) pseudomorph results from cuprite (Cu2O) and malachite (Cu2CO3(OH)2. Loss of constituent is evident in the formation of the pseudomorph.

·       Gypsum (CaSO4.2H2O) is formed from anhydrite (CaSO4) and malachite (Cu2CO3(OH)2) is formed after cuprite (Cu2O)- Gaining of constituents is evident.

·       Goethite (Fe2O3.2H2O) form pyrite (FeS2)-a partial exchange of constituents is evident.

·       Quartz (SiO2) is pseudomorphs after fluorite (CaF2)-a complete exchange of constituents has taken place,

The study of pseudomorphs helps in understanding the evolution of the rocks containing them.