Crystal growth and aggregation

Nucleation (growth) of crystals  Mostly homogenous crystallization from melt or solution  In some cases nucleation on various substrates Epitaxy (a) ○ New mineral overgrowths certain faces and edges of a substrate crystal ○ Orientation of new mineral controlled by crystal structure of substrate Topotaxy (b) ○ New minerals grow on a specific surface such as a fissure vein ○ Orientation of new mineral controlled by orientation relative to the contact surface

Nucleation of crystals  Nuclei – starting point of crystals  Grow further into various crystal morphologies: Combination of crystal forms and aggregation  Crystals can be: Euhedral- perfect polyhedral surfaces - regular and characteristic interfacial angles - in free-space: solution or melt (phenocrysts) - or replacing pre-existing minerals in metamorphic rocks (porhyroblasts) Anhedral- irregular surfaces - in most rocks

Crystal habit  Def: Typical external appearance of a mineral, its combination of crystal forms, and the relative development of these forms  Morphologies mostly not perfect polyhedra  Many minerals have, however, a characteristic shape which is useful for identification Equant or equiaxed Elongated (columnar, prismatic, acicular, fibrous, hair-like) Flattened (platy or tabular)  Can be observed in hand specimens and thin sections as the typical form in which a mineral crystallizes

Crystal habit

Habit  Crystal form depends on growth history If growth velocities remain constant with time, the original morphology is preserved Commonly growth velocities change with time This can be seen best in thin sections of minerals displaying zoning or sector zoning

Twinning  Defintion:Intergrowths where host and twin are sharing a lattice plane (twin plane) or lattice direction (twin axis)  During normal growth of a crystal, lattice layers are added to the crystal face in same orientation as previous ones  In some cases, however, layers added in ‘wrong’ orientation  Twinning begins in most instances when crystals are very small. An ion in the growing crystal may take a position which is not geometrically perfect. If rapid growth takes place, it is possible for this ion to serve as a seed attractive to arriving ions which attach themselves and establish a new direction of growth. Gradually, the offset crystal enlarges and becomes half of the twin.  Since twinning occurs early, it is usual to find twin parts nearly, if not exactly, the same size and exhibiting the same faces, markings, etch pits etc.  Fast growth promotes twinning and conversely slow growth discourages twinning because ions have more time to shift to correct positions.

Types of twinning  Penetration Twins Share a lattice direction (twin axis) or lattice point (twin center). Have an irregular composition surface separating the individual crystals Shown here is a twinned crystal of orthoclase twinned on the Carlsbad Law with [001] as the twin axis.  Contact Twins Share a lattice plane (twin plane or composition plane). These are usually defined by a twin law that expresses a twin plane (i.e. an added mirror plane). The example shown here is a crystal of the mineral gypsum with a contact twin on {100} ○ Swallow Tail Twin Contact twins can also occur as repeated or multiple twins. ○ Polysynthetic twins: ○ The multiple composition planes are parallel to one another. Plagioclase commonly shows this type of twinning, called the Albite Twin Law, with {010} as the twin plane. Such twinning is one of the most diagnostic features of plagioclase. ○ Cyclical twins: ○ If the composition surfaces are not parallel to one another, but related by rotational symmetry, they are called cyclical twins. ○ Shown here is the cyclical twin that occurs in chrysoberyl along a {031} plane.

Twinning

Origin of Twinning  Twinning can originate in 3 different ways: Growth Twins ○ When accidents occur during crystal growth and a new crystal is added to the face of an already existing crystal, twinning can occur The new crystal has to share lattice points on the face of the existing crystal But has an orientation different from the original crystal. Transformation Twins ○ Transformation twinning occurs when a preexisting crystal undergoes a transformation due to a change in pressure or temperature. This commonly occurs in minerals that have different crystal structures and different symmetry at different temperatures or pressures. When the temperature or pressure is changed to that where a new crystal structure and symmetry is stable, different parts of the crystal become arranged in different symmetrical orientations, and thus form an intergrowth of one or more crystals. Dauphiné and Brazil twinning in quartz commonly forms this way during a decrease in temperature. Deformation Twins ○ During deformation atoms can be pushed out of place. If this happens to produce a symmetrical arrangement, it produces deformation twins. The mineral calcite can be easily twinned in this way, producing polysynthetic twins on {012}.

Common twinning laws Isometric System  Spinel Law {īī1} - is a twin plane, parallel to an octahedron. It occurs commonly in mineral spinel (MgAl 2 O 4 ).  Fluorite penetration twin [111] - The twin axis perpendicular to an octahedral face adds three fold rotational symmetry. Most common in fluorite  Iron Cross [001] - The mineral pyrite (FeS 2 ) often shows the iron cross made of the interpenetration of two pyritohedrons.

Common twinning laws Tetragonal System  Twinning in the tetragonal system usually occurs on {011} forming cyclical contact twins.  The minerals rutile (TiO 2 ) and cassiterite (SnO 2 ) commonly show this type of twinning.

Common twinning laws Orthorhombic System  Cyclical Twins {110} - The mineral aragonite (CaCO 3 ), chrysoberyl (BeAl 2 O 4 ), and cerrusite (PbCO 3 ) 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 90 o 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 60 o.

Common twinning laws H exagonal System  Calcite Twins The two most common twin laws that are observed in calcite crystals 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. Brazil Law - {110} - is a penetration twin that results from transformation. Dauphiné Law - [0001] - is also a penetration twin that results from transformation. Japanese Law - {112} - is a contact twin that results from accidents during growth.

Common twinning laws Monoclinic System  Carlsbad Law [001] - forms a penetration twin in the mineral orthoclase. Crystals twinned under the Carlsbad Law show two intergrown crystals, one rotated 180 o from the other about the [001] axis.  Swallow Tail Twins {100}- are commonly observed in the mineral gypsum (CaSO 4. 2H 2 O).

Common twinning 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 ○ Plagioclase (NaAlSi 3 O 8 - CaAl 2 Si 2 O 8 ) very commonly shows albite polysynthetic twinning. ○ The twin law - {010} indicates that the twining occurs perpendicular to the b crystallographic axis. Pericline Law ○ The pericline law has [010] as the twin axis. ○ Pericline twinning occurs as the result of monoclinic orthoclase or sanidine transforming to microcline (all have the same chemical formula - KAlSi 3 O 8 ). ○ Pericline twinning usually occurs in combination with albite twinning in microcline, but is only observable with the polarizing microscope.

Aggregation

Multicrystals, porphyroblasts and poikilocrystals Multicrystals: slightly misoriented individual grains - Quartz quendels - Hematite roses Repeated misorientation leads to forms such as: - Curved surfaces - Saddle morphology Porphyroblasts : When macroscopic crystals grow and replace pre-existing minerals partialy or completely Poikilocrystals: Crystal growing in porous rock to incorporate original rock structure (Sahara Rose – gypsum)

Growth effects  Striations - tourmaline  Indentations  Dislocations and growth spirals – barite  Dissolution (etch pits) - calcite

Isomorphism, polymorphism and crystalline defects

Isomorphism and solid solution  Isomorphism: Similar morphology (crystal forms) Different chemical composition ○ Carbonates:MgCO 3 Magnesite FeCO 3 Siderite ZnCO 3 Smithsonite MnCO 3 Rhodochrosite CaCO 3 Calcite  Solid solution – a special type of isomorphism: Cations replace one another in arbitrary amounts: i.e. minerals can have a composition that is a mixture of two pure “endmember” compositions E.g: Olivine endmembers: Forsterite – Mg 2 SiO 4 Fayalite – Fe 2 SiO 4 Most olivine - (Mg, Fe) 2 SiO 4 Also: Feldspars, Pyroxene

Polymorphism and phase transitions  Polymorphism Different morphology; same chemical composition Depends on external conditions Eg. CaCO 3 - Low pressure: Calcite - trigonal - High pressure: Aragonite - orthorhombic  Undergoes phase transition: Reconstructive – bonds break – totally new structure Order-disorder – atoms are rearranged into more ordered configuration as T decreases Displacive – slight distortion of lattice – no breakage of bonds required

Phase transitions

Crystalline defects  Point defects Vacancy or interstitial atom  Dislocations (line defects) Deformation on slip plane  Planar defects Exsolution at low T Reduction in symmetry  Radiation defects Inclusions: Zircon (U and Th) in biotite ○ Haloes of different colors ○ Destruction of crystal (metamictization)

Crystalline defects  Point defect  Dislocations (Linear defect)

Planar defects  Exsolution and ordering