ARTICLE:GEOLOGY TWINNING IN OPTICS

GEOLOGY TWINNING IN OPTICS

TWINNING occurs when two separate optics share some of the same optics lattice points in a symmetrical manner. The result is an intergrowth of two separate optics in a variety of specific configurations. A twin boundary or composition surface separates the two optics.

Crystallographers classify twinned optics by a number of twin laws. These twin laws are specific to the optic system. The type of twinning can be a diagnostic tool in mineral identification.

Twinning can often be a problem in X-ray crystallography, as a twinned optic does not produce a simple diffraction pattern.

TYPES OF TWINNING

Simple twinned optics may be contact twins or penetration twins. Contact twins share a single composition surface often appearing as mirror images across the boundary. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning.

Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. An example is metazeunerite.

In penetration twins the individual optics have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite, pyrite, and fluorite often show penetration twinning.

 

 

Galvanized surface with macroscopic crystalline features. Twin boundaries are visible as striations within each crystallite, most prominently in the bottom-left and top-right.

If several twin optic parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called polysynthetic twins.

When the multiple twins are not parallel they are cyclic twins. Albite, calcite, and pyrite often show polysynthetic twinning. Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the optic face. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern.

 

 

 

Twinned optics (e.g., see Figure ) may be described as follows:

Simple twins – composed of only two parts
Multiple twins – composed of more than two orientations
Contact twins – this occur if a definite composition plane is present

Penetration twins – occur if two or more parts of a optic appear to interpenetrate each other with the surface between the parts being indefinable and irregular

(Figure ).

COMMON TWIN LAWS

  • TRICLINIC SYSTEM

The feldspar minerals plagioclase and microcline are the most common triclinic minerals that show twinning.

Two common twin laws are observed in these feldspars.

 

 ALBITE LAW

As described above, plagioclase (NaAlSi3O8 – CaAl2Si2O8) very commonly shows albite polysynthetic twinning.  The twin law – {010} indicates that the twining occurs perpendicular to the b crystallographic axis. Albite twinning is so common in plagioclase, that it’s presence is a diagnostic property for identification of plagioclase.

 

 PERICLINE LAW

o                        The pericline law has [010] as the twin axis.  As stated above, pericline twinning occurs as the result of monoclinic orthoclase or sanidine transforming to microcline (all have the same chemical formula – KAlSi3O8).

o                      Pericline twinning usually occurs in combination with  albite twinning in microcline, but is only observable with the polarizing microscope.  The combination of pericline and albite twinning produce a cross-hatched pattern, called tartan twinning, as discussed above,  that easily distinguishes microcline from the other feldspars under the microscope.

 

  • MONOCLINIC SYSTEM

                         The most common twins in the monoclinic system occur on the planes {100} and {001}.  The feldspars – orthoclase and sanidine – are the most commonly twinned minerals in the monoclinic system.  Both contact twins and penetration twins occur, and both types result from accidents during growth.

 

 MANEBACH LAW – {001} –

             It forms a contact twin commonly observed in the mineral orthoclase. This twinning is very diagnostic of orthoclase when it occurs.

 

 CARLSBAD LAW – [001] –

o                     It forms a penetration twin in the mineral orthoclase. Optics twinned under the Carlsbad Law show two intergrown optics, one rotated 180o from the other about the [001] axis. Carlsbad twinning is the most common type of twinning in orthoclase, and is thus very diagnostic of orthoclase when it occurs.

 

 BRAVENO LAW – {021} –

It forms a contact twin in the mineral orthoclase.

 

 SWALLOW TAIL TWINS – {100}-

They are commonly observed in the mineral gypsum (CaSO4.2H2O).

 

·     ORTHORHOMBIC SYSTEM

                     Orthorhombic optics commonly twin on planes parallel to a prism face.  The most common is a {110} twin that results in many orthorhombic minerals having cyclical twins.

 

 {110} CYCLICAL TWINS –

The mineral aragonite (CaCO3) , chrysoberyl (BeAl2O4), and cerrusite (PbCO3) commonly develop twinning on {110}.  This results in a cyclical twin which gives these minerals a pseudo-hexagonal appearance.

  STAUROLITE LAW

               The mineral staurolite is really monoclinic, but it has a ß angle very close to 90o so it has the appearance of an orthorhombic mineral.  Two types of interpenetration twins occur in staurolite the {031} twins from a right-angled cross and the {231} twins form a cross at about 60o.

 

  • TETRAGONAL SYSTEM

Twinning in the tetragonal system usually occurs on {011} forming cyclical contact twins.  The minerals rutile (TiO2) and cassiterite (SnO2)  commonly show this type of twinning.

 

  • HEXAGONAL SYSTEM

The minerals calcite (CaCO3) and quartz (SiO2) are the most common hexagonal minerals and both show the types of twinning common in hexagonal minerals.

 

 CALCITE TWINS

o                  The two most common twin laws that are observed in calcite optics are {0001} and the rhombohedron {012}.  Both are contact twins, but the {012} twins can also occur as polysynthetic twins that result from deformation.

Quartz shows three other hexagonal twins.

o    BRAZIL LAW – {110} –

o                 It is a penetration twin that results from transformation.

o    DAUPHINÉ LAW – [0001] –

o             It is also a penetration twin that results from transformation.

o    JAPANESE LAW – {112} –

o               It is a contact twin that results from accidents during growth.

  • ISOMETRIC SYSTEM

Three types of twins are common in the isometric system.

 

 

o    SPINEL LAW – {1} –

It is a twin plane, parallel to an octahedron.  It occurs commonly in mineral spinel (MgAl2O4).

 

 [111] – The twin axis perpendicular to an octahedral face adds three fold rotational symmetry.
 IRON CROSS [001] –

The mineral pyrite (FeS2) often shows the iron cross made of the interpenetration of two pyritohedrons.  Since this occurs in the class 2/m, with no 4-fold rotation axes, the [001] twin axis gives the mineral apparent 4-fold symmetry about 3perpendicular axes.

POLYSYNTHETIC TWINNING –

Occurs when three or more individuals are repeated alternately on the same twinned plane. If the individuals of polysynthetic twins are thin plates, the twinning is called lamellar e.g. plagioclase feldspars.

 

MODES OF FORMATION

There are three modes of formation of twinned optics. Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. Annealing or transformation twins are the result of a change in optics system during cooling as one form becomes unstable and the optics structure must re-organize or transform into another more stable form.

Deformation or gliding twins are the result of stress on the optics after the optics has formed. If a FCC metal like aluminum experiences extreme stresses, it will experience twinning as seen in the case of explosions. Deformation twinning is a common result of regional metamorphism.

 

Optics that grow adjacent to each other may be aligned to resemble twinning. This parallel growth simply reduces system energy and is not twinning.

 

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