Isostructural Minerals

Two or more minerals are said to be isostructural when their atoms are arranged in the same type of crystal structure - the same geometry of bonding, the same coordination polyhedra, the same symmetry - even though their chemical compositions are entirely different. ^[1] Isostructuralism is sometimes called isomorphism or, less commonly, isotypism, but neither of these terms is recommended because they carry other meanings in different contexts. ^[1]

Why Isostructuralism Occurs

Isostructural relationships arise because crystal structure is controlled primarily by the geometry of ion packing - the coordination numbers, radius ratios, and bond strength rules described by Pauling’s Rules - rather than by the specific chemical identity of the ions involved. If two different pairs of ions happen to have similar radii and the same charges, they will adopt the same structural framework. The result is two minerals that may have almost nothing in common chemically, yet share an identical arrangement of atoms in space.

Halite and Galena: The Classic Example

Halite (NaCl) and galena (PbS) provide a textbook example: the three-dimensional spatial distribution of lead and sulfur in galena precisely mirrors the geometric arrangement of sodium and chlorine in halite. ^[1] In physical and chemical terms the two minerals could hardly be more different - halite is a colorless, water-soluble salt; galena is a dense, opaque, grey sulfide ore of lead. Yet the fact that they share the same structure produces identical symmetry, identical cleavage (three perfect cleavages at right angles), and identical crystal habit (cubes) in both minerals. ^[1] This is what makes isostructuralism mineralogically significant: structural identity produces identical symmetry-dependent properties regardless of chemical identity.

Isostructural Groups

An isostructural group is a set of minerals that are both isostructural with each other and chemically related - they share a common anion or anionic group, with the cation substituting between members. ^[1] The calcite group of carbonate minerals is the most widely cited example. All members crystallize in the same point group (3¯2/m), carry the carbonate (CO₃^2-) anionic group, and differ only in which divalent cation occupies the cation site: ^[1]

Within an isostructural group, the different cations can substitute for one another to form solid solutions, because the structural framework accommodates ions of similar size and charge. The physical properties of the group members - cleavage, symmetry, crystal habit - are shared, while properties that depend on which specific cation is present, such as color, density, and hardness, vary between members.

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