Petrography as a science began in 1828 when Scottish physicist William Nicol invented the technique for producing polarized light by cutting a crystal of Iceland spar, a variety of calcite, into a special prism, which became known as the Nicol prism. The addition of two such prisms to the ordinary microscope converted the instrument into a polarizing, or petrographic microscope. Using transmitted light and Nicol prisms, it was possible to determine the internal crystallographic character of very tiny mineral grains, greatly advancing the knowledge of a rock's constituents. It is a branch of geology that can be applied to concrete or construction raw materials, which examines and evaluates the optical properties and micro structural characteristics of raw materials.
During the 1840s, a development by Henry C. Sorby and others firmly laid the foundation of petrography. This was a technique to study very thin slices of rock. A slice of rock was affixed to a microscope slide and then ground so thin that light could be transmitted through mineral grains that otherwise appeared opaque. The position of adjoining grains was not disturbed, thus permitting analysis of rock texture, thin section petrography became the standard method of rock study. Since textural details contribute greatly to knowledge of the sequence of crystallization of the various mineral constituents in a rock, petrography progressed into petrogenesis and ultimately into petrology.
The basement or Precambrian terrains, which are invariably metamorphosed, tectonically disturbed and affected by intrusive plutons are some of the potential targets for crystalline rocks, which can be applied in construction.
Crystalline basement rocks are used widely in a variety of applications including road construction, buildings, pavements, dam fills and a range of other uses. However, the effective use of crystalline basement rocks for construction purposes depends on a range of factors. Some of these factors include mechanical strength, physical strength index (e.g. porosity, dry density and specific gravity) and petrographic characteristics (e.g. mineral grain size, mineral interlocking relationship, quartz, feldspar and mica content)
Natural construction aggregate is one of the most abundant natural resources and one of the most widely used. Construction aggregate is the sized, or crushed and sized, rock material used in concrete and asphalt, which make up most of highways, bridges, houses and other engineering works.
More than 90% of asphalt pavement and 80% of concrete consist of construction aggregate. The remainder is a binder such as asphalt or cement. About 52% of all construction aggregate is crushed stone, while 48% of the remaining is sand and gravel (Bragg, 1986).
Selecting the right aggregate material is imperative to overcome the frequent problem of pavement failure. In the various ways in which aggregate is used, it is exposed to a variety of stresses, and the response of the structure in which it is used will largely depend upon the properties of the aggregate. It needs to resist heavy loads, high impacts and severe abrasion, and it needs to be durable in the prevailing environmental conditions. In general, they should be hard and tough enough to resist crushing, degradation, and disintegration from any associated activities.
In assessing the potential of a rock for use as an aggregate, the first requirement is a full petrographic study of the rocks, identifying its mineralogy, grain size, texture, fabric (sum of textural and structural features), and the weathering states which are determined by the geological processes that formed the rock. These processes decide the ways in which a rock may fail, and also help to optimize production of good-quality aggregates.
Petrographic examination of potential construction aggregates deposits originates in the field with an onsite visual examination of the aggregate source, and is finished in the laboratory using petrographic method .Field investigation for a rock may consist of rock identification (rock types), degree of weathering (fresh, slight, moderate, and high) and recording of any geologic feature as faults joints cleavages (Bragg, 1989).
Any deposit that is being considered as a construction aggregate should be initially rated on the basis of the amount of deleterious substances present. Deleterious substances are those materials or features which occur in the rock or the rock surface that are capable of causing adverse effects resulting in premature detrition of the rock, some of these materials are clays, organic matter, mica, and sulphides.
Petrographic examination should identify and call attention to potentially alkali-sillica reactive and alkali carbonate reactive constitute, determine such constitutes quantitatively and recommend additional test to confirm or refute the presence in significant amounts within the aggregate. Alkali-silica reactive constitutes found in construction aggregates include high strained quartz, tridymite, chalcedony. Aggregate materials containing these constituents are phyllits, gneiss, schist, gneissic granite. Therefore, selecting aggregates with the necessary characteristics for a particular site is imperative. Increase in road construction projects in Nigeria demands the location and development of suitable quarries that will provide aggregates close to their end market. This is because long haulage distance from existing production areas are not only cost-prohibitive and infeasible, but also initiate early deterioration or failure of existing roads which did not factor in heavy duty vehicular traffic during their design.
The Ajaokuta area lies between latitude 7˚20'N and 7˚35'N and Longitude 6˚30'E and 6˚44', and is within the south-western Basement Complex of Nigeria. The Nigerian Basement Complex consists of the migmatite-gneiss complexes, the older metasediments, the younger metasediments, the older granites and, the younger granite complexes and volcanic rocks. The Migmatite Gneiss Complex consists of gneisses, amphibolites, migmatite and metavolcanics. The older metasediments include rocks such as quartzite, calcareous rocks (e.g. marble) and relics of highly altered clayey sediments and igneous rocks. The Younger Metasediments consists of quartz-biotite-muscovite schist and coarse-grained feldspar-bearing micaceous schist although ferruginous quartzite and talc schist may also occur. The older granites occur as large circular masses within the Schist and the Migmatite-Gneiss Complexes. The Younger Granite Complexes formed a distinctive group of intrusive and volcanic rocks bound by ring dykes or ring faults. Locally the rock types located in the field are quartzite, porphyritic granite, migmatite, mica schist, biotite gneiss (Aidetomre et al, 2014). This study has its rationale as assessment on the usability of crystalline rocks as construction aggregates.
The aim of this study attempts to carry out petrographic and mineralogical studies of various rock unit obtained within the study area through thin section microscopy.
The study area lies within Ajaokuta local government in Kogi State and lies within latitude 0134 47 4N and longitude 08 37 39°E respectively. The study area consists of both tarred roads and footpaths leading to the outcrops. The Ajaokuta local government was created from the Okene Local Government on 27th August 1991 and has its headquarters in Agodo. The Local Government is in the central senatorial district of the state and covers a land mass of 1362 square kilometers. The Ajaokuta local Government is bounded to the north east by Lokoja Local Government Bassa Local Government to the North West, Ofu Local Government ot the east and south West by Okene and Adavi local government area respectively. The outcrops were accessible through road and foot paths, due to the dry season conditions vegetation were sparse due to lack of sufficient rainfall as a result thorough study was carried out which was possible due to the dry season conditions.
A through geologic field exercise was carried out as an initial part of the study using: Topographic maps, compass clinometers and Global position systems (GPS), at each location fresh samples were collect at exposed outcrops along road cuts and other areas along the area of study. Afterwards they were taken for laboratory analysis.
The study area can be classified as a rural area both private houses, estates and company quarters and all situated among the Ajaokuta main town. Ajaokuta local government is populated with lots of commercial and industrial activities. The settlements are nucleated the main occupations are crop farming, animal husbandry and petty trading. The land available is being used by both private and commercial farmers.
Ajaokuta is characterized by two distinct seasons which are defined by a period of rainfall and a period of dryness, the rainy season begins in April and ends in October while the dry season begins in November and ends in March. Temperature prevailing in the area is generally high with values ranging from 240’c to 35crc with an annual mean of 3Ocrc, the average rainfall is about 250mm, the study area lies within the middle belts of Nigeria which is characterized by Guinea- Savannah type vegetation comprises of their forest clusters of trees, shrubs and grasses (Adegbe et al., 2014).
The study area is characterized by several exposed outcrops and low-land, the major river in Ajaokuta is the river Ero which empties into the river Niger. The drainage pattern is said to be dendritic with NW-SW direction.
A number of investigations have studied the relationship between petrographical and mechanical properties of granites. The results of these investigations indicated that the mechanical strength of granites are generally a function of a wide range of petrographic parameters including grain size (Eberhardt et al., 1999, Tugrul and Zarif, 1999, Akesson et al., 2001, Prikryl, 2006 and Yilmaz et al., 2010), mineral composition (Miskovsky et al., 2004), and weathering (Tugrul, 2004, Vasconcelos et al., 2008 and Basu et al., 2009). Although the relationships between the mechanical properties and petrographical characteristics of granitic rocks are well known, the effect of mineralogical characteristics on the engineering properties of granite types is different.
The total amount of mica in a rock is in itself not as important as foliations defined by micas (Lindqvist et al., 2007).The strength of a rock undergoes a notable reduction on weathering (Bell, 2007). Generally the alteration product of plutonic rocks has high clay content. Clays are mechanically weak and their abundance has a deleterious effect on the durability aspects of a rock (Lindqvist et al., 2007).
A number of techniques were developed by Willard and McWilliams (1969) in an attempt to gain a better understanding of the mechanical behavior of rocks in relation to their microstructure (petrographic characteristics). They reported that microfractures, grain boundaries, mineral cleavages and twinning planes influence the ultimate strength of a rock and may act as surfaces of weakness, which control the direction in which failure occurs. The strength of a rock is related to the presence of discontinuities, where cracks can be initiated at the time of failure (Lindqvist et al., 2007).
Lindqvist et al., 2007 described that fine grained rocks varieties are generally stronger than the coarse grained ones. It is not only the grain size but also the grain size distribution that is important, with the effect that a large size range gives higher strength and better resistance to fragmentation and wear compared to a more equigranular or idioblastic rock .Euhedral mineral grains may act as discontinuities where cracks may initiate in the structure (Lindqvist et al., 2007). On the other hand, an increased complexity in the grain shape and grain boundary geometry increases the strength of the material. This may be due to subgrains occurring at the grain boundaries. Going from an idioblastic texture, in which the shape of the grains is largely defined by straight surfaces, to xenoblastic texture with more irregular grain boundaries, increases the strength and resistance to mechanical fragmentation (Åkesson et al., 2003).