EASA Mineralogy Homepage

Week 5: Feb. 13 and 15

order-disorder polymorphism - changes in crystal structure due to ordering and disordering of ions in the lattice. (Example - Potassium feldspar, K[AlSi3O8], can occur as the polymorphs sanidine (high T) and microcline (low T). When Al and Si are randomly distributed in the tetrahedral sites then one gets monoclinic symmetry (sanidine), when there is ordering of the Si and Al then the symmetry becomes triclinic (microcline) (see fig. 4,12 of your book and the handout I gave you)

Pseudomorphism - the existence of a mineral (called the pseudomorph) with an outward crystal form of another mineral (ex. limonite pseudomorphs after pyrite' graphite pseudomorphs after diamond). Mechanisms are:

Substitution - removal of one ion and replacement by another (ex. SiO2 pseudomorphing wood; petrified wood).
Encrustation - formation of casts of one mineral by another
Alteration - formation of a new mineral after another by the partial removal or addition of material (usually the addition of water). Example - pseudomorphs of serpentine after olivine
Polymorphic transformations - (see above). Example - alpha quartz pseudomorphing bi-pyramidal beta quartz

schotkky defect - ion missing from its normal site. (If a cation is missing then an anion may be missing as well or a cation with higher valence occurs nearby to maintain charge balance)
Frenkel defect - removal of an ion from its normal site and emplacement at an interstitial site. (More common for cations than anions because of their smaller size)
Impurity defect - addition of a "foreign" ion at an interstitial site or as a replacement of a normal ion (Example, Cr replacing Al in corundum, Al2O3, (forms red ruby), and Fe-Ti replacing Al-Al forming blue sapphire)

Line defects - concentrations of defects along a line. Also called "dislocations" because they cause offsets in crystal structure. Dislocations are important for the understanding of crystal deformation--they facilitate deformation by reducing the energy needed for deformation. Work hardening or strain hardening occurs when disclocations interfere with each other or are removed from the crystal by deformation.

Movie of an edge dislocation, viewed on one face only, moving through a lattice leading to a net change in shape of the lattice. Note that bonds are broken and reformed one at a time only. Here is another one. And another one showing the lattice planes only.

Movie of a screw dislocation passing through a lattice resulting in change in shape of the lattice.
Movie of a combined screw dislocation and edge dislocation traversing a lattice.

Plane defects - two dimensional zones along which slightly displaced "blocks" within a crystal are joined.

subgrain boundaries or lineage structures - boundaries between parts of a grain that are slightly rotated with respect to each other.
stacking faults - regular sequence of atomic layers is disrupted by a slight shift in an adjacent layer; the two parts are nor rotated with respect to each other.

Twinning - the symmetrical intergrowth of two or more crystals of the same mineral (called "twinned crystals")

Twins are related by a symmetry element that is absent from the single crystal (otherwise you just propagate the same crystallographic orientation--no twinning)
Possible symmtery elements for twinning:

gives the symmetry operation and its orientation
gives the orientation of the "composition surface" (the surface along which the twins are united; if planar then "composition plane")
example: The twin law for albite twins (which are almost always present in the mineral plagioclase) states that the twin axis is perpendicular to (010), and the composition plane is {010}.

contact twins - planar surface between twins
penetration twins - interpenetrating twins with an irregular composition surface
repeated or multiple twins - three or more twins
cyclic twins - multiple twins that have non-parallel composition planes
polysynthetic twins - multiple twins with parallel composition planes (ex. albite twinning is usually polysynthetic)

Growth twins (or "primary twins") - accidents of growth; usually simple penetration twins (ex. Carlsbad twinning in orthoclase)
transformation twins - due to polymorphic transformations involving a change in symmetry (ex. "tartan twinning" in microcline after transformation from orthoclase)
deformation twins (or "glide twins") - stress causes a portion of a crystal to twin relative to the rest of the crystal (ex. very common in calcite)

Compositional variation in minerals - recall that minerals have a "definite range in chemical composition"; this recognizes that minerals can have variable compositions within limits.

Solid solution - a mineral that displays variability in chemical composition due to substitution of different ions. Solid solution also refers to the phenomenon of ionic substitution (also called "diadochy")
example: olivine is a solid solution between forsterite (Mg2SiO4) and fayalite (Fe2SiO4).
Major factors affecting solid solution:

ionic size of substituting ion - a good rule of thumb is that if the size difference between the replacing ion and the original ion is <15% then complete or nearly complete solid solution occurs. If it is greater than 15% then there is limited or no substitution.
charge - there must be overall charge balance in the crystal so the charge of the substituting ion must be the same as the original ion or there must be other substitutions (or omissions/additions) of ions for charge balance (see later)

example: in plagioclase, which is a solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), when Ca+2 replaces a Na+1 ion, an Al+3 must replace a Si+4 to maintain charge balance.

simple - one ion replaces another (ex. see olivine above)
coupled - one pair of ions replaces another pair to maintain charge balance (ex. see plagioclase above)

Omission solid solution - a more highly charged cation "replaces" two or more lower charged ions creating vacancies

example - in microcline, KAlSi3O8, a single Pb+2 may replace two K+1 ions resulting in a vacancy: K + K = Pb + vacancy. (This vacancy causes the green color of some microcline samples--"amazonite") Note: this generates a "shotkky" defect, which we covered earlier

example: tremolite has the formula rCa2Mg5[Si8O22](OH)2, where r denotes a vacant site. Na+1 may fill this site while an Al+3 replaces a Si+4 to retain charge balance giving NaCa2Mg5[AlSi7O22](OH)2, which is called edenite.