Lecture 7

Classification

First, to continue where we left off last time …

Last week we discussed physical properties of minerals, particularly those related to the interaction of light with the mineral surface or interior. Now I’d like to briefly discuss some other physical properties that you’ve had a chance to use in lab.

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 also 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)

Classification of Minerals

While there are many different ways that one could classify minerals (color, shape, association, etc.), you can already guess that mineralogic classification systems are based primarily on composition and structure. We use a classification system that is little changed from that originally proposed by Dana, and assign minerals to classes based on either their anions or anion complexes. Thus the mineral class is related to the mineral formula. It also means that minerals within the same class tend to have similar structures, and thus similar physical properties. Finally, minerals in a single class are often found in association with one another.

Common mineral classes and example minerals are listed in Table 2.5 of your text. Of these, the silicates are the most abundant, and are subdivided on the basis of the structure of the silica tetrahedral (see below). Other common mineral classes (and ones that you have seen or will be seeing in lab) include

halides [with anions Cl^-, F^-, Br^-, I^-]

oxides [with anion O^2-]

hydroxides [with anion complex OH^-]

carbonates [with anion complex CO₃^2-]

native elements

sulfides [with anion S^2-]

Classification of silicates

In most silicates, Si⁴⁺ exists in 4-fold coordination with O^2-. The subclasses are classified according to how the tetrahedral are linked (see Table 2.6 in your text); because the linkage determines the number of Si per O, each subclass has its own distinctive Si:O ratio. The most common subclasses of silicates are:

Framework silicates (including quartz and feldspar, the most abundant elements in the Earth’s crust); Si:O ratio is 2:1, although in many framework silicates Si is replaced to some extent by Al (as in the feldspars). Framework silicates are subdivided by groups shown in Table 2.7

Sheet silicates – this group includes serpentines, clays and micas. Sheet silicates consist of sheets of SiO4- tetrahedral (arranged as joined 6-fold rings) separated by octahedral layers that contain cations (commonly either Al³⁺ or Mg²⁺). The Si:O ratio in these minerals is 2:5 (which often appears in the mineral formula as 4:10). The tetrahedral and octahedral layers can then be stacked in different ways – for example, serpentine and kaolinite have alternating T and O layers, while pyrophyllite and talc have TOT sequences that are loosely joined to each other by van der Waals bonds (hence the softness of talc)

Single chain silicates – the single chain silicate group contains all of the pyroxenes, with structures based on chains of SiO4- tetrahedral linked by shared (often called bridging) oxygens. The Si:O ratio is 1:3 (often written as 2:6). The most common pyroxenes involve solid solutions between Mg, Fe and Ca, but other forms may include Na (jadeite) and Li (spodumene). In the image below diopside is the pale blue mineral … more commonly, however, it is green. Chrome diopside is a beautiful dark green gemstone.

Jadeite is one mineral known as jade, although nephrite (an amphibole) is also called jade (and tends to be a darker green in color).

Double chain silicates – these structures are intermediate between the pyroxenes and the sheet silicates in having linked chains of tetrahedral, separated by octahedral layers. The characteristic Si:O ratio is 4:11 (8:22). This group includes all of the amphiboles and the “pyroxenoids”.

Isolated tetrahedra – this group contains some important minerals, many of which you already know. First there are the olivines, with Si:O ratios of 1:4. Also in this group are the aluminosilicates (sillimanite, kyanite, and andalusite) with fairly invariant formulas of Al₂SiO₅. Finally, there are some other distinctive minerals such as staurolite (whose name means “cross”), titanite (CaTiSiO₅), topaz, and zircon (ZrSiO₅).