State of Aggregation

Oxygen, nitrogen and carbon dioxide are examples of natural gases; and water, mercury and oil are examples of natural liquids. With the exception of mercury and the natural mineral oils, all the minerals we deal with are found naturally in the solid state, and the properties dependent on their state of aggregation are now considered. 

Form

Under favourable conditions minerals assume a definite crystal form. The requirements are that the crystal has been free to grow outwards into the solution or melt from which it formed, neither obstructed by other solid matter nor hindered by a shortage of the constituents needed for growth. In such an environment, the mineral develops as a crystal with a regular pattern of faces and angles between adjoining faces which are character-

istic of a particular mineral. The study of this regularity of form, and of the internal structure to which it is related, is called crystallography, and is dealt with in the next chapter. The following general descriptive terms are

associated with the crystal characters of minerals : 

Crystallized = A term denoting that the mineral occurs as well developed crystals. Most of the beautiful mineral specimens in museums are of crystallized minerals.

Crystalline = A term denoting that no definite crystals are developed, but that a confused aggregate of imperfect crystal grains have formed, interfering with one another during their growth. 

Cryptocrystalline = A term denoting that the mineral possesses traces of crystalline structure.

Amorphous = A term used to describe the complete absence of crystalline structure, a condition common in natural rock glasses but rare in minerals. 

Habit 

The development of an individual crystal, or an aggregate of crystals, to produce a particular external shape, depends upon the conditions during formation. One such environment may give long needle-like crystals,

whereas another may produce short platy crystals; and it is quite possible for the same mineral to have several different habits. Descriptive terms for mineral habits are split into (a) those for individual crystals, and (b) those for aggregates of crystals, and these are now described. 

Individual crystals 

Acicular = Fine needle-like crystals, as in natrolite. 

Bladed = Shaped like a knife blade or lath-like; a form commonly displayed by kyanite. 

Fibrous = Consisting of fine thread-like strands, as shown by the variety of gypsum called satin-spar, and also by asbestos. 

Foliated or foliaceous = Consisting of thin and separate lamellae or leaves, as is shown by the mica group minerals and other sheet silicates.

Lamellar = Consisting of separable plates or leaves, as with wollastonite. 

Prismatic = Elongation of the crystal in one direction, as in the feldspars, the pyroxenes and the common hornblendes,

Reticulated = Crystals in a cross-mesh pattern, like a net, as in rutile needles found within crystals of quartz.

Scaly = In small plates, as in tridymite.

Tabular = Broad, flat, thin crystals, as in wollastonite and sanidine feldspar.

Crystal aggregates 

These may be aggregates of crystals, of which individuals can be seen with the naked eye; or massive aggregates of minerals in which individual crystals are too small to be seen with the naked eye: 

Amygdaloidal = Almond-shaped aggregates, common in the zeolites, in which the minerals occupy vesicles or gas holes in lava flows.

Botryoidal = Spherical aggregations resembling a bunch of grapes, as in azurite and prehnite. 

Columnar = Massive aggregates in slender columns, as is seen in stalactites and stalagmites, usually with the mineral calcite. 

Concretionary and nodular = Spherical, ellipsoidal or irregular masses, as in flint nodules.

Dendritic and arborescent = Massive aggregates in tree-like or moss-like shapes, usually with, the mineral being deposited in crevasses or narrow planes, as with the dendrites of manganese oxide.

Granular = Coarse or fine grains. Evenly sized granular aggregates of minerals, such as olivine in the ultrabasic rock dunite, are often termed saccharoidal because of their resemblance to lumps of sugar.

Lenticular = Flattened balls or pellets, shown by many concretionary and nodular minerals.

Mammilated = Large mutually interfering spheroidal surfaces, as in malachite.

Radiating or divergent = Fibres arranged around a central point, as in barite and in many concretions. 

Reniform = Kidney-shaped, the rounded outer surfaces of massive mineral aggregates resembling those of kidneys, and perfectly displayed by the variety of hematite called kidney-iron ore.

Stellate = Fibres radiating from a centre to produce star-like shapes, as in wavellite.

Wiry or filiform = Thin wires, often twisted like the strands of a rope, as in native silver and copper . 

Pseudomorphism 

Pseudomorphism is the assumption by a mineral of a form belonging to another mineral. Pseudomorphs may be formed in several ways:

(1) By investment or incrustation, produced by depositing a coating of one mineral on to the crystals of another; for example, a coating of guartz on fluorite cyrstals. 

(2) By infiltration, when the cavity previously occupied by one mineral is refilled by deposition in it of a different mineral by the infiltration of a solution. 

(3) By replacement, from slow and gradual substitution of particles of new and different mineral matter for the original mineral particules which are removed by solution. This type of pseudomorphism differs from the preceding one in that substitution takes place before the previous mineral has vacated the space it occupied. 

(4) By alteration, due to a gradual chemical change which crystals sometimes undergo, their composition becoming so altered that they are no longer the same minerals, although they possess the same forms; for example, the alteration of olivine to serpentine. 

Pseudomorphs may often be recognized by a lack of sharpness in the edges of the crystals, while their surfaces usually have a dull and somewhat granular or earthy appearance. 

Polymorphism 

Two or more minerals may possess quite different physical properties, such as colour, form, hardness, specific gravity, etc., and yet may have identical chemical compositions. Such minerals represent a series of polymorphs. In a polymorphous series of minerals the atomic lattices are different so that their physical properties also differ. Good examples of polymorphs are calcite and aragonite (dimorphous minerals); while graphite and diamond, which exhibit different forms of crystals, differ in hardness and specific gravity, and have markedly different optical properties. The three Al2SiOs polymorphs, andalusite, kyanite and sillimanite, also have different physical properties; for example, their crystal forms differ and their specific gravities or densities are 3.13-3.16, 3.58-3.65, and 3.23-3.27 respectively; and their optical properties are also quite different. From experimental phase equilibria investigations, it is known that each of the three polymorphs exists under different conditions of temperature and pressure. High-density polymorphs, such as kyanite and diamond, are invariably favoured by high-pressure conditions, and therefore great depth. 

Polytypism 

Polytypes are minerals with the same chemical formulae and the same structural sub-units, which are stacked in different ways. Thus there are numerous polytypes of silicon carbide, depending upon the number of

close-packed repeat layers. Aristotypes are related to polytypes and include the sheet silicates, which show different stacking arrangements of sheets of atoms, with the general formulae [Si40 101f or the silicate layers,

and either [Al(OHhl or [Mg,Fe(OHhl for the linking sheets. These sheets are stacked along the crystallographie c axis, to produce minerals in which