Structural Defects

The idealised model of a crystalline structure assumes a perfectly regular, infinitely repeating pattern of atoms or ions that continues without interruption throughout an entire crystal. Real crystals, however, always deviate from this ideal. Every real crystal contains a variety of irregularities - locations where the pattern is broken, distorted, or interrupted - and these structural defects are not rare exceptions but universal features of all crystalline materials. Defects influence a wide range of properties, from electrical conductivity and colour to mechanical strength and diffusion rates, making them directly relevant to understanding how minerals behave under geological conditions. ^[1]

Structural defects are grouped into three categories based on their geometry: point defects, which are localised at a single atomic site; line defects, which extend along a one-dimensional path through the crystal; and planar defects, which occupy a two-dimensional surface. ^[1] As a general rule, the density of defects - that is, the number of defective sites per unit volume - is higher in crystals that grew at high temperature, because high temperatures favour disordered atomic arrangements. ^[1]

Point Defects

Point defects are localised irregularities at individual atomic sites within the crystal structure. They take four forms: vacant sites, misplaced atoms, foreign atoms in normally empty sites, and foreign atoms substituting for normal constituents.

Schottky Defects

Schottky defects are vacancy defects - sites that should be occupied by an ion are instead empty. ^[1] In an ionic crystal, electrical neutrality must be maintained throughout the structure, so cation vacancies cannot exist in isolation. For every vacant cation site there must be a corresponding vacant anion site to keep the charge balance intact. In a simple structure such as NaCl, this means that every vacant Na⁺ site is accompanied by exactly one vacant Cl^- site. ^[1] In minerals with more complex compositions - where cations and anions carry different charges - the ratio of vacant cation sites to vacant anion sites will differ from 1:1, but the total charge removed by the vacancies must still sum to zero. Crucially, because cations and anions are removed in charge-compensating pairs, Schottky defects do not alter the ratio of cations to anions. The stoichiometry of the mineral is unchanged. ^[1]

Frenkel Defects

Frenkel defects are mislocation defects - an atom or ion is displaced from its normal crystallographic site and relocated to an alternative site that is not normally occupied. ^[1] Cations are far more commonly involved than anions in Frenkel defects, because cations are smaller and can more easily migrate between sites without severely disrupting the anion framework around them. ^[1] Because the displaced ion is still present in the crystal - it has simply moved - no net charge is added or removed from any region of the structure. Frenkel defects therefore do not affect charge balance or the stoichiometry of the mineral. They do, however, create two local disturbances simultaneously: a vacancy at the original site and an interstitial occupant at the new site. ^[1]

Impurity Defects

No mineral is chemically pure in the strict sense of containing only the elements in its ideal formula. Other elements are always present, even if only in trace amounts, and the ways in which these foreign elements are accommodated in the structure define two types of impurity defect. ^[1]

An interstitial defect occurs when a foreign atom or ion occupies a site that is normally vacant - a gap or cavity in the structure that the ideal mineral does not use. ^[1] Because inserting an ion into a site introduces charge, electrical neutrality must be restored elsewhere in the structure - for example, by replacing a higher-charge cation with a lower-charge cation somewhere nearby to compensate for the charge introduced by the interstitial. ^[1]

A substitution defect is the simpler case: a foreign ion directly replaces one of the ions that is normally present in the crystal structure, occupying the same site. ^[1] The boundary between a substitution defect and what the solid solution literature calls ionic substitution is one of degree: when foreign ions are present only in trace amounts and are not part of a systematic solid solution series, mineralogists treat them as substitution defects rather than as evidence of a solid solution.

The identity of any impurity defect depends on first establishing what counts as the pure composition and ideal structure for that mineral. High temperatures allow more impurities to be accommodated because thermal vibrations are greater at high temperature, enabling atoms and ions to occupy positions that would be structurally inaccessible to them at lower temperature. ^[1]

Schottky and Frenkel defects were first proposed to explain how atoms and ions diffuse through crystal structures. For an ion to move through a solid, it must either jump into a vacant site - a Schottky defect - or temporarily occupy an interstitial position, forming a transient Frenkel defect. The abundance of both defect types increases with temperature, and this is precisely why diffusion of atoms and ions through crystalline solids is faster at high temperatures: more available sites and more frequent thermal kicks make migration easier. ^[1]

Line Defects

In tectonic environments that combine high temperatures and pressures with slow deformation, rocks deform in a ductile manner rather than fracturing. Folds ranging from microscopic to mountain-range scale are the visible result, and for them to form the individual mineral grains making up the rock must themselves deform. Studies of ductile deformation in crystalline metals reveal that crystals deform by slip - the displacement of one part of the crystal relative to another along favoured crystallographic planes in specific crystallographic directions. ^[1] The crystallographic plane on which slip occurs combined with the direction of slip defines the slip system. In a simple orthorhombic lattice, for example, the slip system {001}[010] means that slip takes place on planes parallel to {001} in a direction parallel to [010] - the b crystal axis. ^[1]

Producing slip by breaking all chemical bonds along the slip plane simultaneously would require orders of magnitude more energy than experiments actually show is needed. The explanation is that slip does not work that way. Instead, bonds break only along a single advancing line - the dislocation line - which marks the boundary between the portion of the crystal where slip has already occurred and the portion where it has not yet occurred. Only the bonds at this line are breaking at any given moment, which is why the energy requirement is so much lower. ^[1] There are two geometrically distinct varieties of dislocation line.

Edge Dislocations

In an edge dislocation, the Burgers vector - the vector that quantifies the magnitude and direction of slip - is oriented at right angles to the dislocation line itself. The Burgers vector is identified experimentally by tracing a Burgers circuit: starting at a lattice node S, one traces a rectangular path taking equal numbers of lattice steps in opposite directions. In a perfect crystal this circuit closes exactly; when it encloses a dislocation it fails to close, finishing at a point F. The vector from F back to S is the Burgers vector. With progressive deformation, the edge dislocation line moves through the crystal in the direction parallel to the Burgers vector. ^[1]

Screw Dislocations

In a screw dislocation, the Burgers vector is oriented parallel to the dislocation line rather than perpendicular to it. Applied stress produces a shear parallel to the dislocation plane and parallel to the Burgers vector, but the dislocation line itself moves at right angles to the Burgers vector as deformation progresses. Tracing a Burgers circuit around a screw dislocation would define a spiral - like the threads on a screw or the steps of a spiral staircase - which is the geometric origin of its name. ^[1] Screw dislocations play an important dual role in mineralogy: they form during deformation, but they also facilitate crystal growth by providing a continuously available step onto which successive layers of atoms can be added (as discussed in the Crystal Growth page).

Geometry and Consequences

Dislocation lines cannot simply end inside a crystal - both ends of a dislocation line must either terminate at the crystal surface or the line must form a continuous closed loop within the crystal. Along its length, the character of a dislocation can change from edge to screw type, but the Burgers vector remains constant throughout and is always parallel to the sense of shear that created the dislocation. A dislocation is geometrically identical to a fault in structural geology - the Burgers vector is analogous to the slip vector, and the dislocation line corresponds to the fault trace. ^[1]

Mineral samples that have been strongly deformed typically contain very high densities of both edge and screw dislocations. When dislocations in different orientations interfere with each other and cease to propagate, the material becomes harder and more resistant to further deformation - a process called work-hardening, familiar from the behaviour of metals bent repeatedly at the same point. Heating the deformed material provides enough thermal energy for the crystal structure to rearrange itself and remove the dislocations, restoring ductility in a process called annealing. ^[1]

Planar Defects

Planar defects involve a mismatch in the crystal structure across a surface. The surfaces may be perfectly planar or curved in complex ways. Three types are most important in mineralogy: grain boundaries, stacking faults, and antiphase domain boundaries. ^[1]

Grain Boundaries

In the mathematical ideal, a crystal’s regular atomic pattern continues to infinity in all directions. In reality, every crystal is finite and its lattice terminates somewhere - at a crystal face, at a fracture or cleavage surface, or at the boundary where one mineral grain meets another. ^[1] These termination surfaces are grain boundaries, and because the crystal pattern is abruptly interrupted at them, they represent higher-energy regions of the structure. This elevated surface energy at grain boundaries is precisely what drives grain growth during recrystallisation - the system lowers its total energy by reducing the total amount of grain boundary area.

Stacking Faults

Stacking faults form in layered structures when one layer is inserted in the wrong position or wrong sequence. In hexagonal closest packing the layers alternate between positions A and B to give the repeating pattern ABABABAB. In cubic closest packing a third position C is used to give ABCABCABC. A stacking fault would be present if a C-position layer were inserted into an otherwise hexagonal close-packed structure, producing a sequence such as ABABCABAB - one layer is out of its normal order, and the local stacking sequence is disrupted at that point. ^[1] Minerals that display polytypism - including the micas and clay minerals - are particularly prone to stacking faults when their regular repeating layer pattern is disrupted. ^[1]

Antiphase Boundaries

Antiphase boundaries separate regions of a crystal called antiphase domains. Adjacent domains have identical crystallographic orientations - they are not rotated or reflected relative to each other - but there is a translational mismatch between them: one domain is offset from the other by a simple translation vector. This means the pattern in one domain is the same as the neighbouring domain but shifted, so that what is an A site in one domain falls on a B site in the other. ^[1]

Antiphase boundaries are most commonly produced when a crystal undergoes a displacive polymorphic transition. Pigeonite, (Mg,Fe,Ca)₂Si₂O₆, provides the classic example. At high temperature, the pyroxene chains of silicon tetrahedra in pigeonite all have the same geometry. When the temperature falls and pigeonite undergoes its displacive transition, alternate layers of chains kink in opposite directions - one layer kinks inward and the adjacent layer kinks outward. In an ideal crystal this alternating pattern would be consistent throughout the entire grain. In practice, however, one domain adopts one kinking pattern and an adjacent domain adopts the opposite pattern, creating an antiphase boundary between them where the two patterns meet. ^[1] Because adjacent antiphase domains share the same crystallographic orientation, they cannot be distinguished in hand specimens or by conventional microscopic techniques. Imaging by transmission electron microscopy reveals that antiphase domains are typically highly irregular in shape. ^[1]

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