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• Structure in Quarrying Granite
• Mineralogically Granite
• Origin of Granites
• Forms of Igneous Rocks
• Texture in Quarrying Granites
• Mineralogy and Chemistry
• Granite - Conclusion

Texture in Quarrying Granites

Structures are large scale features present in outcrop, while the textures are not visible by naked eye, it can be seen only under microscope. Vesicular and Amygdaloidal structures are commonly found in basalts (commercial name: black granite). Most lavas are heavily charged with gas, the escape of the gas distends the molten material with the production of cavities, bubbles or vesicles which may be spherical, elliptical, cylindrical or regular in shape. Amygdales are the infillings of vesicles by secondary minerals like calcite, silica, zeolites or indefinite hydrated ferro-magnesium silicates. So, rock with these structures is not advisable for cutting and polishing, because these infillings may come out after some time.

Structures characterised by fracture or parting of rock mass are joints, sheets and platy structures. These are very useful in quarrying granites. In granites it is common to find three sets of joints, one more or less horizontal, the other two vertical and perpendicular to each other. If these three systems of partings are equally spaced, the fracture planes give rise to a structure of cuboidal blocks. These types of joints are known as 'mural joints' (fig. II-a).

Rift and Grain: In quarrying granite advantage is taken of mural jointing to procure large blocks; but in dressing the blocks down to smaller dimensions, quarrymen make use of rift and grain (Fig. II-b), which are directions of comparatively easy splitting at right angles to one another, one horizontal and other vertical. In the directions at right angles to both the rift and the grain, the granite breaks with a rough irregular fracture which is called the 'hard way', the 'tough way' or the 'head'. Rift is the direction of easiest fracture, the 'cleaving way' or 'reed'. Along the grain the 'qartering way' or the 'hem', fracture is less perfect. The perfection of rift and grain determines the ease with which kerb stones and setts can be made.

According to T.N. Dale rift and grain structures consists of minute cracks from 0.09 - 1.30 mm apart, crossing the quartz particles and extending into the feldspars. These cracks determine the rough fissibility in two rectangular directions of which the usually horizontal rift is the more pronounced. These fractures coincide with, or are parallel to bands of fluidal cavities in the quartz grains. These sheets cross one another at right angles, and those in the grain directions are the less abundant. Both sets are independent of sheet or flow structures in the granite, but the rift is frequently parallel to mica flakes and to the long axes of feldspar phenocrysts when these are present. These structures may be present in other igneous rocks also as granite, but usually to much less extent than in granite.

The production of rift and grain is dependent on the crustal stress which were in operation at the time of consolidation of the granite. These sheets of fluidal cavities were formed when quartz, was crystallising. Under the same stress the mica plates became aligned in directions roughly parallel to the rift. Then as crustal stresses continue to operate after the rock has solidified, the rift and grain (cracks) were produced, following the planes of weakness developed by the cavities. Hence the study of these directions may provide a valuable information to quarrymen, even before procurement of granitic blocks. These also give an idea of the nature and direction of the earth forces which were concerned in their production.