The seven most important physico-chemical processes and principles that have now been used to define the origin of rocks and mineral deposits are: -  
 

1) The precursor principle.

The concept that most common ore and rock minerals crystallised by simple dehydration reactions from hydrous minerals of corresponding composition (such as feldspars from clays) was developed by Professor R.L. Stanton during his studies of Broken Hill metamorphic rocks 1978 to 1983. He then measured the composition of the substances that were crystallising to metamorphic minerals with a microprobe analyser. He established the importance of this geological process as the precursor principle in his 1989 review paper. It has never been disputed that crystalline metamorphic rocks and granites were originally derived from sediments but classical theory assumes the sediments were lithified to lower-energy refractory rocks and rock minerals that then required massive later inputs of energy to liquefy or re-crystallise them. Stanton has been first to determine the ordered precursor hydrates from which metamorphic minerals actually crystallise.
STANTON, R.L., 1989. The precursor principle and the possible significance of stratiform ores and related chemical sediments in the elucidation of processes of regional metamorphic mineral formation. Phil. Trans. R. Soc. Lond., A 328: 529-646.

 
 

2) Diffusion gradients.

The removal of ions, molecules, macromolecules, or charged particles from dispersion or solution in pore fluid by physical or chemical processes such as adsorption, coagulation, concretion, or chemical reactions reduces the concentration of the species involved at and near the reaction site. This depletion of a particular species at the site of a reaction or interaction creates a concentration gradient along which more dispersed units of the same species diffuse to equalise their concentration. Diffusion is a phenomenon well known to all scientists. The concept of ions, molecules or very small particle diffusing at different rates through porous media was introduced by a Scottish chemist, Thomas Graham in 1829. He formulated Graham’s Law for the diffusion rate of gases through porous media, which varies inversely with the square root of their densities.
He also proposed the term ‘colloid’ to distinguish dispersions of particulate matter from what he called ‘crystalloids’ that are now regarded as electrolytes or ordinary solutes.
GRAHAM, T., 1864. On the Properties of Silicic Acid and Other Analogous Colloidal Substances. (See Alexander, A.E., and Johnson, P., 1950. Colloid Science. Oxford University Press, p.5.)

 
 

3) Accretion.

This process is the rapid formation of clusters of similar shaped particles to form ‘close packed’ and pre-ordered aggregates at net lower surface energy. The formation of accretions is dynamic. It occurs in any remobilised fluid paste where particles of each colloidal component have similar or the same shapes that enable them to be packed closely together. There is a critical concentration of particles at or above which close packing occurs. Crystallisation of these pre-ordered aggregates subsequently occurs to then form a ‘porphyroblastic’ texture where the large crystals are set in a finer grained matrix of crystallised sedimentary material. The development of accretions as a general principle is demonstrated by their occurrence in formerly re-mobilised sediments and lodes of many different types and compositions. They occur in greywackes, shales, limestone, banded iron formations, slip complexes, spotted shales, sheared zones, granites, rapakivi granites, porphyroidal “clay clot” rocks, porphyroids, jasper, chert, porcelanite, deep marine oozes, sandstone, dolomite, marble and greenstone.
Professor A.E. Alexander, Department of Physical Chemistry, University of Sydney, identified cherty ‘porphyroblasts’ or ovoids from the intrusive porphyroid lenses at Black Angel in Tennant Creek, N.T., as accretions in 1959. He pointed out that rapid formation of physically stronger aggregates of close packed particles was due to the operation of van der Waal’s forces at close interparticle spacing in accord with DLVO theory and he gave as references:
ALEXANDER, A.E., and JOHNSON, P., 1946. Colloid Science. Vol 2, Clarendon Press, Oxford, England, p. 607.
FRISCH, H.L., and SIMHA, R., 1957. The Viscosity of Colloidal Suspensions and Macromolecular Solutions. In Rheology, Theory and Applications (ed. F.R.Eirich), Vol.1, pp. 525–613. Academic Press, New York.
HAUSER, E.A. and LeBEAU, D.S., 1941. Studies in Colloidal Clays; II. Jour. Phys. Chem., 45: 54-64.
MIELENZ, R.C. and KING, M.E., 1955. Physical-Chemical Properties and Engineering Performance of Clays. Calif. Dept. Nat. Resources, Bull. 169, pp.196-294.

 
 

4) Concretion.

This is the slow or step–wise accumulation of material about a central nucleus to produce a banded–textured spherical or elliptical accumulation of higher particle density and compaction than the medium in which the particles are diffusing. The active process of concretion depends on colloidal particles individually diffusing towards the precipitating surface represented by the boundary of a higher density gel aggregate within the less dense surrounding medium through which the particles are diffusing. They form by diffusive migration of dispersed sol particles to the surface of a contracting gel aggregate. Precipitation at this 'internal' surface is a consequence of a marked change in electrolyte concentration that is produced by synerectic exudation of the interparticle fluid from the densifying gel. Concretions require a synerectic nucleus on which the metastable sol particles can precipitate. The additional layers of precipitated particles then in the 'close packed' condition, draw together under van der Waal's strong attractive forces to desorb their own adsorbed species so that the concretion continues to grow. Concretion is therefore a diffusive process dependent on the concentrations of electrolytes and smaller particles dispersed within the gel meshwork. It does not depend on the operation of external agencies such as shearing. The characteristic form of polyrimmed concretions is an important and reliable indication of the environment in which these structures are formed. They are complex but definite non-random structures that can only be formed in aqueous particle systems.
MIELENZ, R.C. and KING, M.E., 1955, were first to point out that the synerectic contraction of a gelatinous aggregate resulted in the exudation of more highly concentrated electrolytes than that in the surrounding pore fluids. However, concretions as sub-spherical accumulations of one of the very fine grained constituents of the rock in which they occur have been well known and described for over a century. An early reference is:
DALY, R.A., (1900) The calcareous concretions of Kettle Point, Lambton County, Ontario, J. Geol., vol. 8, pp. 135-150.

 
 

5) Synerectic desorption.

This is one of the most important properties of close-packed particle aggregates (accretions and concretions). As the particles in these clusters are drawn together by van der Waal’s strong forces of attraction at very close interparticle separation (primary minimum), the particles or particle chains achieve greater co-ordination. Total surface energy is lowered and internal surface and adsorptive capacity are reduced. Species adsorbed on surfaces are desorbed. Polar water molecules, ions, and smaller charged particles are exuded from the clusters into the matrix brines. This results in Liesegang banding, rimming of synerectic aggregates (rapakivi texture and concretion), and discharge to the pore fluid brines of exceedingly small metal hydroxide and hydroxy-sulphide particles. Precipitates of these metastable colloids from brines migrating out of the system accumulate as economic orebodies. Syneresis in accretionary aggregates is independent of the chemical composition of the particles. Therefore, any large volume of limestone, chert, argillaceous sediment (shale), or greenstone that has been re-textured is a potential source of ore mineral sols. These dispersions seep out of the compacting sediment pile as synerectic desorption develops brine strengths that will keep them displaced into the fluid phase.
In 1861, Thomas Graham identified the spontaneous contraction of colloidal aggregates and dense gels and introduced the term “syneresis” to describe this process. The desorption of ions and polar water molecules that result from reduction of surface area within close-packed synerectic gels was identified later following the work of Freundlich, 1936, and the development of DLVO theory in the 1940’s.
FREUNDLICH, H., 1936. Structures and Forces in Colloidal Systems. Proc. Royal Inst., 29: 232–252.

 
 

6) Critical cluster.

Analyses of the processes involved in nucleation and crystal growth shows that initiation of the solid phase is achieved by chemical reaction between ions or molecules that would lead to the formation of a solid precipitate. Where the reacting substances are at low concentration or the reaction is controlled by diffusion rates, this leads to the formation of a critical cluster or nucleus that has special properties at the point where a liquid-solid interface is first formed. The orientation of insoluble molecules in relation to each other as a crystal lattice or nucleus begins to form results in “dangling bonds” or unsatisfied chemical linkages where additional molecules could join the developing lattice. The extraordinarily high surface energy of such nascent nuclei or critical cluster of molecules is sufficient to dissociate water molecules. The “dangling bonds” are satisfied with H+ or OH- to form hydroxide or hydroxy-sulphide particles.
Critical clusters are centres from which spontaneous growth can occur. This growth results in the formation of a stable charged particle with a surface but the initial growth of exceedingly small and sparsely separated charged particles means that the charged hydroxide and hydroxy sulphide particles are, like the sparsely positioned metal ions from which they were formed, again adsorbed on sediment substrate surfaces. Hydroxide, hydroxy-sulphide, metal, and gangue mineral species are successively released (paragenetic sequence) by ion - charged particle exchange as the concentration of pore fluid brine increases with the depth of burial and progressive diagenesis. The exceedingly small size of the dispersed ore metal sulphide particles enables them to pass through the smallest openings and diffuse out of the system with the fluids.
The concept and special properties of a ‘critical cluster’ as a first stage in the formation of a precipitate by a chemical reaction was introduced to this research by Professor T.W. Healy in 1972. It has since been reproduced in other textbooks on physical chemistry.
HEALY, T.W., 1972. Physico–Chemical Processes in the Diagenesis of Sediments. Unpublished Paper, Dept. of Physical Chemistry, University of Melbourne, Vic. p. 13.
STUMM, W., 1992. Chemistry of the solid-water interface: Processes at the mineral-water and particle-water interface in natural systems. A Wiley-Interscience publication, New York, p. 212.

 
 

7) Rheological separation.

This is an important principle by which components of gelatinous materials such as semi-consolidated sediments separate by differences in their fluid properties. When the gel structure or meshwork is physically disrupted or disturbed the more mobile components simply “flow out” of the agitated or disturbed pastes. They are less viscous, water-rich and therefore lighter or more “sloppy” components of the mixed hydro-silicates. They tend to form veins or dykes that intrude upward. Iron hydroxides and hydrous ferromagnesian minerals such as chlorite, glauconite, or serpentinite apparently retain water of hydration later in diagenesis than clays and silica gels. However, late separation and injection of polymeric silica globules as water-rich thixotropic liquid is common. The separation usually occurs at a stage where the other major components would re-assume a non-fluid or more viscous gel condition.
Rheological separation can involve large volumes of the mobilised pastes or occur on a very small scale. Fluid hydrous chlorite oozing from flow sheared Conasauga shale into microscopically small ptygmatic veins has been illustrated by Weaver (Fig. 5.35 in the e-book "The Origin of Rocks and Mineral Deposits"). Glauconite dykes and sills have separated from tertiary limestone near White Rock in New Zealand (illustrated in Figures 13.20 to 13.26). The separation of the hydrous precursor of high-grade iron ore from disturbed banded iron formations is an important economic result of rheological separation (Figures 6.2 and 8.33).
WATERHOUSE, J.B., and BRADLEY, J., 1957. Redeposition and slumping in the Cretaceo-Tertiary strata of S.E. Wellington. Trans. Roy. Soc. N.Z., 84: 519-594.
WEAVER, C.E., 1984. Shale-slate Metamorphism in Southern Appalachians. Elsevier, Amsterdam. Developments in Petrology 10, p. 49.