Lecture 6

Crystal growth, Physical properties of minerals

Crystallization

Crystallization involves nucleation of a “seed” crystal and subsequent growth of that crystal. Nucleation involves competition between the supersaturation driving crystallization and the surface energy created by formation of a new phase. For this reason, high supersaturations (a large driving force) promotes nucleation. In contrast, once nuclei exist, they may grow at smaller supersaturations.

The figure below illustrates the different supersaturation regimes anticipated for different locations of magma cooling. Slow cooling (low supersaturation) of rocks within the Earth’s crust is often invoked to explain the lower number density, but larger size, of crystals in plutonic rocks relative to volcanic rocks that cool on the Earth’s surface.

Differences in nucleation and growth behavior can also explain the difference between the salt and alum crystals that you grew in lab….

Crystal growth

Processes of crystal growth aren’t perfect… for example, crystals that grow rapidly may develop skeletal or dendritic forms. Such a crystal is shown in the photo on the right. This is a scanning electron microscope image where backscattered electrons are collected to yield an image that tells us about composition. You can see many different features in this image. First, the crystal appears to have a hole in the middle … this is the texture that we call skeletal. Second, note that the structures at the corners of the crystals are “decorated” with the onset of dendritic overgrowths. Finally, note that the crystal is zoned in gray scale, which represents compositional zoning (discussed below).

And most crystals have some sort of imperfection (many of which are diagnostic of that crystal). Which leads to a discussion of:

Crystal imperfections - defects

Defects important in that they increase crystal reactivity ...

Point defects

All crystals above absolute zero contain some defects ... increases energy of system, thus more at high temperatures

1. Impurity defect – results from the presence of a foreign atom, either replacing one normally in the structure or filling a “vacancy”.

2. Paired vacancies Anion vacancies are regions where there is more positive charge - may trap nearby electron ... transitions between energy levels may be invisible range - color center For this reason, can induce colors using radiation (Pleichroic halos around zircon inclusions) Vacancies are important for process of diffusion, that is, moving ions through the crystal structure.

3. Line defects - happen when rock is stressed. Most easily understood with reference to simple cubic lattice. dislocations - extra plane of atoms.

edge dislocation – occur when a plane of atoms in a structure terminates at a line in the crystal’s interior

screw dislocation - caused by displacement of part of crystal structure one translation unit relative to another such that the displacement terminates along a line perpendicular to growing face.

4. Stacking faults – may separate layers that are out of order

Crystal imperfections – zoning

Compositional zoning occurs when different parts of a mineral have different compositions (through various substitution mechanisms). This is illustrated nicely in the olivine picture above, where the dark gray part of the crystal is enriched in Mg, and the bright part of the crystal is rich in Fe.

Compositional zoning is particularly common (and diagnostic) in plagioclase. The crystal shown on the left shows several different types of zoning, including

normal zoning (the general outward trend from brighter, Ca-rich plagioclase to darker, Na-rich plagioclase)

oscillatory zoning (the fine scale structure that oscillates between slightly darker and slightly lighter shades of gray)

sector zoning (which is illustrated by the faint diagonal bands that open outward from the crystal’s interior

Crystal imperfections - twinning

Twins result when different domains of a crystal structure have different orientations. They share atoms along a surface – typically a composition plane. You can think of these as mistakes in where the next plane of atoms is placed (think of stacking the clear plastic spheres …

Descriptions of types of twinning:

simple twins – if composed of only two parts, that is, if the twins can reflect across a single plane

multiple twins – refers to twins of multiple orientations

penetration twins – describes the growth condition when two or more parts of a crystal appear to penetrate each other.

Most common (and distinctive) are the twinning habits of feldspars. Plagioclase feldspar is triclinic, thus there are no planes of symmetry to control twin planes. Twinning in plagioclase consists of numerous repeated twins, called polysynthetic twins. Most spectacular when viewed in thin section.

Physical properties of minerals

We’ll look at two different aspects of physical properties – those that are important for diagnostic identification of minerals (mostly “scalar”), and those that dictate the physical behavior of minerals (those that often show directionality, that is they are “vector” properties).

HAND SPECIMEN PROPERTIES

As these properties are best learned in lab, I will just present an overview in class.

1. Appearance

LUSTER – general appearance or sheen … examples include metallic, vitreous, adamantine (diamond-like). Metallic luster is the result of near-complete reflection of light by the mineral surface. The adamantine luster of diamond is a consequence of its high index of refraction

DIAPHENITY – refers to a mineral’s ability to transmit light (transparent, translucent, opaque). Most opaque minerals have metallic luster.

COLOR – often useful for quick ID (particularly when color is distinctive), but can be very misleading. Color is controlled by “chromophores” and is a consequence of the interaction of light with electrons in the crystal.

Allochromatic minerals have color caused by elements that are present in trace amounts, like the Cr that causes the green color of beryl to the right (emerald), or the Ti that gives corundum the blue that we call sapphire.

In contrast, idiochromatic minerals have color as an intrinsic property, sometimes on that changes with solid solution composition, and thus may be diagnostic not only of mineral type but also end member composition (as in garnet). Examples of idiochromatic minerals include Cu-bearing minerals (which are typically blue or green) and Mn-bearing minerals, which are typically pink. Color can also be created by electron vacancies to form “color centers” (particularly common in fluorite).

STREAK – the color of finely powdered mineral; useful for distinguishing oxides and sulfides

LUMINESCENCE – any emission of light that is not the direct result of incandescence; includes properties such as fluorescence and phosphorescence. Luminescence of minerals is another property that may be controlled by trace amounts of an element.

COLOR PLAY – refers to properties of light scattering, as seen in the “star sapphire” in the picture. In this case the “star” of light is created by light scattering from small inclusions that are arranged along the three principle crystallographic directions.

Other examples of color play include the iridescence that is characteristic of labradorite; here the scattering is the result of very fine-scale exsolution.

Opalescence is probably one of the best examples of color play – opalescence is the result of silica precipitation as tiny spherical bodies that are able to scatter light.

2. Crystal shape

Called crystal “habit” – the appearance of minerals, either as single crystals or as aggregates; includes terms such as the fibrous growth of cerussite (Pb carbonate) crystals to the left, or the botryoidal habit of smithsonite (Zn carbonate) to the right.

3. Strength – related primarily to bonding

TENACITY – cohesiveness, or resistance to breaking. Terms to describe tenacity include brittle (ionic bonding); malleable (metallic bonding), flexible (characteristic of sheet silicates like mica)

CLEAVAGE, FRACTURE, PARTING – reaction of crystal (strain) to an external force (stress). “Cleavage” is the tendency of minerals to break along certain planes (EX: graphite). When minerals break along planes of weaknes they have “parting”; weakness may be twinning, pressure solution. When minerals do not have a dominant plane of weakness they “fracture” in patterns that may be described as “conchoidal”, “fibrous”, “hackly”.

HARDNESS – resistance of a smooth surface to scratching. Hardness is probably a consequence of weakest bond in structure.

4. Density (specific gravity)

Ratio of the weight of a substance and the weight of an equal volume of water. Determined as

Specific gravity (the ratio) can be measured by a Jolly balance; density requires a pycnometer. The density can be calculated from the mineral formula if you know the dimensions of the unit cell and the number of formula units per unit cell.

5. Magnetism

Magnetite and pyrrhotite are the only common minerals with a magnetic signature.

ANISOTROPY AND PHYSICAL PROPERTIES

Many physical properties are anisotropic, such that their magnitude depends on the direction in the crystal. An easy way to picture this is by a mechanical analogue with springs of different stiffness in different directions… net displacement is the result of the vector sum of the components, thus the direction of displacement is not necessarily the same as the direction of the applied force.

DIRECTIONAL PROPERTIES

thermal conductivity relateds heat flow to temperature gradient

electrical conductivity relates electrical current density to electric field

diffusivity relates atomic flux to concentration gradient

elastic properties relate strain (extent of deformation) to applied stress

seismic properties relate to velocity of seismic wave propagation (related to density, rigidity, bulk modulus)

optical properties relate to refractive index variations

Examples: calcite shows double refraction

ulexite is a natural fiber optic

Each of these properties is controlled by the crystal structure, such that

• the directional variation in the value of a physical property must be consistent with the point group symmetry of the crystal

• since physical properties can always be broken into three mutually perpendicular components, the symmetry of physical properties may be greater than the symmetry of the crystal itself

Physical properties may be

isotropic – uniform in all directions (isometric crystals)

uniaxial – similar in two directions and different in the third (hexagonal and tetragonal crystals)

biaxial – different in all three directions (orthorhombic, monoclinic, trigonal crystals)