Sample Science Paper on Petrological and Mineralogical Processes


History is full of information regarding rock formation and the processes that leads to
their formation. According to Wager, Brown & Wadsworth (1960), among the most common
types of rocks are the igneous rocks that forms layered intrusions. These rocks have recorded
the oldest one more that 4.2 and distributed in various locations and are vertically layered
making them have varying compositions and textures. This can also be attribute to their
petrological processes and the mineralogical processes. Consequently, Zhou eta al. (2005)
described layered intrusion as large sill-like body composed of igneous rocks with sizes range
of 100 km 2  (39 sq mi) to over 50,000 km 2 . Consequently, Large layered intrusions are formed
by igneous rocks mainly ultramafic to mafic in composition that flows and solidifies forming
layers due to time of the magma flow and different mineral content of the lava.

Difference between petrological and mineralogical processes
According to Zipfel et al. (2000), Petrology is term that is used to denote the study of
rocks such as igneous, among others and the associated processes that lead to their formation
and transformation. Among them are the layered intrusions that are formed win this process
due to successive crops of crystals due to cooling magma. It is during this process that due to
heat loss through the boundaries of cooling magma makes a growing crystal to exerts
gravitational stress on the liquid suspending it. The crystal will ten sinks or float at a rate
guided by properties of the liquid crystal. Also, due to increased rate of temperature loss and
solids leads to increases the viscosity and increases strength of the crystal-liquid suspension.
Therefore, if stress exerted by the crystal becomes greater than the yield strength of the
liquid, it means that crystal will continues to move but if the rate of growth and density
contrast are high, the result is rapid sinking or a floating enough to escape or become trapped
in the solidifying matrix.

Petrological and Mineralogical Processes

During this process, most of the heat is known to move from intrusions to the roof
making crystallization be concentrated on the floor. As Jackson (1961) has pointed out,
however, the positive pressure dependence of crystallization temperature implies that a
homogeneous magma should crystallize more rapidly at its base than under its roof. Any
concentration of volatile components near the top will augment this effect (Sorenson, 1969;
Elsdon, 1970). However, there is presence of dynamic processes during crystallization.
Among the most common includes differential settling or flotation of crystals with varying
densities and grain sizes. This affects flow of crystal-laden magma and crystal separation
during convective fluid movement into the lava compartment. Other processes that contribute
to these formations include magma injection into the compartment leading to magma mixing
and formation of silicate liquid immiscibility in the chambers.
However, non-dynamic processes may be involved which includes rapid changes in
crystallization conditions creating disruptions in the process curves. Formations of the layer
may also originate form variation in nucleation proportions and from mineralogy due to rock

Petrological and Mineralogical Processes
heterogeneous properties. It is also known that majority of these processes are driven by
degeneracy of energy making them self-organization processes to form modal layers.
Consequently, Dana (1869) defined mineralogy as the study of associated chemistry,
crystal structure and physical properties of rock mineral constituents. Therefore, it is
important to consider the petrological and mineralogical processes involved as they affect the
environmental. As a demonstration of the process, it is known that dark minerals have higher
density than light ones like pyroxene is ca. 30% denser than plagioclase and settles more
rapidly to form a layer with dark minerals and vice versa forming contrasting layers. Hence,
according to Jerram, Cheadle & Philpotts (2003), crystal-sorting mechanism is the main
cause of magma differentiation conditions, so the compositions of rocks, and the minerals
they consist of, are interrogated to answer fundamental questions across a wide range of
geological disciplines.

Crystal Fractionation

Fractional crystallization, or crystal fractionation, is one of the most important
geochemical and physical processes operating within crust and mantle of a rocky planetary
body, such as the Earth. It is important in the formation of igneous rocks because it is one of
the main processes of magmatic differentiation. Crystal settling may occur in a surprisingly
diverse range of regimes and may lead to intermittent deposition events even with small
crystal concentration. It is caused by continuous separation of crystals and liquid as crystals
solidifies. Consequently, more minerals in the crystals and the liquid causes change of liquid

Petrological and Mineralogical Processes
contents per unit of crystallization. The liquid composition is varied by presence of major
elements in crystals and liquid.
Therefore, the nature and proportions of the crystallising phases and relative
concentrations in the liquid of elements that enter into solid solutions is also a major factor.
Fractional crystallisation of basaltic liquids usually involves separation of olivine, pyroxene
and plagioclase. Consequently, Gravitational settling occurs which results to graded bedding.
This is a result of denser crystals settling at the bottom of the magma body and become
segregated from the residual melt. The densest particles will settle at the bottom of the
magma chamber. The process can also be supported by Stokes Law of residual magmatic

Double Diffusive Convection

As result of this varying properties, double diffusive convection becomes apparent
which then yields to rhythmic layering that cause repetition of zones of varying composition
as the large layered intrusions.

Cyclic layering

During formation of large layered intrusions, cyclic layering is differentiated by
remarkable regular spacing of the layers with geometric increase in spacing. In particular,
layers are always seen to be parallel to the contact. The phenomenon of cyclic layering is to
magma chamber recharge. In this, there is continuous depletion in compatible trace minerals
form base to top representing the composition of the initial magma. The latter is maintained
until a new base is attained which signifies an injection of fresh magma where mixing can
occur leading to a mixture with a different composition.

Petrological and Mineralogical Processes

Density Currents

As a result of varying mixtures properties of gases and liquids, motion is maintained
by the force of gravity that acts in differences of density leading to modal layers.

Magmatic Segregation

This is a general term that refers to all the process responsible for more minerals
getting locally concentrated during the processes of magma cooling and crystallization. As a
result, the resultant rocks formed are magmatic cumulates. Therefore, despite the initial
magma being homogeneous liquid, it can become a complex mixture along the path. It is also
evident that magmatic cumulates mineral deposits are strictly in mafic and ultramafic igneous
rocks due to silica exerting control exerted on the viscosity of a magma i.e. high silica content
means more viscous a magma slowing down segregation proceed.

Bagnold effect

It is known to be the most important in determination of flowage differentiation due to
is consideration of buoyancy. However, this phenomenon is absent in porphyritic rocks. For
large channels, magmatic differentiation that exists between the core and the walls does not
relate to the forced lava flow. However, the latter is determined by other factors like crystal
settling among others.

Large layered intrusions

From the Zhou eta al (2005) description of layered intrusion, these rocks can be
termed as composed of both heterogeneous and homogeneous masses of rock. Their
formation is as result of two or more types of rocks with seamless connections between the
rock types which gives the impression that different magmas were involved in their formation
but did not mix before solidifying. In most cases, these rocks conform to the floor of the

Petrological and Mineralogical Processes
intrusion region but varies in thickness and length. It may be explained by the fact that the
processes are dominant in these areas over varying period of time among other factors. Due
to varying composition and mixture, the resultant layers of rocks have different texture and/or
varying mineral proportions.

Layering in the Skaergaard intrusion: Source (Wager, L. R., and Deer, W. A. (1939)
Layered intrusions demonstrate a repetition of patterns in the layers giving an impression of a
cycle of conditions leading to their formation. In many cases, the large layered intrusion
contacts are planar with a characteristic of eroded hollows in some underlying layer having
been filled by an overlying layer showing a repeated process of formation. The spreading of
the magma as it flows causes it to cover a large area and as these processes’ repeats, they
form a Large layered intrusion

Examples of these intrusions

Among the best examples of large layered intrusions includes the Macro-rhythmic
layering in Greenland. In this case, the formation demonstrates sequences with initial layers
being enriched with Fe–Ti oxides, clinopyroxene-rich gabbro and plagioclase-rich layer in
the same order. A second example is the micro-rhythmic layering in the Bjerkreim in
Norway. In this case, successive layers are highly enriched with orthopyroxene and oxide
minerals followed by plagioclase. A third example is the very-fine scale layering in the

Petrological and Mineralogical Processes
Storgangen intrusion in Norway. The fourth example is the modally-graded layering in the
magnetite layer in South Africa. The feature demonstrates continuous increase of plagioclase
mode upwards. These are some of the known examples among many that exist.

Petrological and Mineralogical Processes

Dana, J. D. (1869). A system of mineralogy.
Gibb, F. G. (1992). Convection and crystal settling in sills. Contributions to Mineralogy and
Petrology, 109(4), 538-545.
Jerram, D. A., Cheadle, M. J., & Philpotts, A. R. (2003). Quantifying the building blocks of
igneous rocks: are clustered crystal frameworks the foundation? Journal of
Petrology, 44(11), 2033-2051.
Wager, L. R., Brown, G. M., & Wadsworth, W. J. (1960). Types of igneous
cumulates. Journal of Petrology, 1(1), 73-85.
Zipfel, J., Scherer, P., Spettel, B., Dreibus, G., & Schultz, L. (2000). Petrology and chemistry
of the new shergottite Dar al Gani 476. Meteoritics & Planetary Science, 35(1), 95-
Zhou, M. F., Robinson, P. T., Lesher, C. M., Keays, R. R., Zhang, C. J., & Malpas, J. (2005).
Geochemistry, petrogenesis and metallogenesis of the Panzhihua gabbroic layered
intrusion and associated Fe–Ti–V oxide deposits, Sichuan Province, SW
China. Journal of Petrology, 46(11), 2253-2280.