GEOLOGY Subject : GEOLOGY (For under graduate student.) Paper No. : Paper 02 Introduction to Geology 02 Topic No. & Title : 42 Texture of Igneous Rocks (Part 01) Academic Script Introduction The first attempt to catch the petrological study of a rock always begins with the serious study of thin sections-to study the nature of minerals and to analyze mutual relations of these minerals. No petrological study can be complete without an understanding of textural and structural development of the rock. Interpretation of textures and microstructures is important to understand evolution of rocks. The texture of a rock is the result of various processes that controlled the rock genesis and along with mineralogy and chemical composition. Some confusion exists and many authors do not distinguish clearly between these three concepts. Which are texture, structure and fabric. Texture Texture refers to specific inter-mineral relationships that may be repeated throughout the rock. Structure Structure has two meanings when applied to igneous rocks: (i) it can refer to certain large scales features such as block lava, pillows etc or (ii) it denotes small scale features (microstructure) that affect the rock. These include colour banding, flow banding and joints etc. 1
Fabric Fabric is overall aspect of the rock, which results from a combination of textural and structural elements. Fabric is influences by overall texture together with the shape of components. The five basic parameters are required to describe the texture of a rock. These are as follows: a)colour b)crystallinity the degree of crystallization c)granularity the absolute size of crystals d)crystal shape e)texture the mutual relations of crystals (or crystals and glass) a) Colour Every rock sample should be classified according to colour (presence of dark minerals) using the following scheme: Leucocratic - < 30% dark minerals Mesocratic - 30 to 60% dark minerals Melanocratic - > 60% dark minerals This is purely a reflection of minerals colour and is independent of the subdivision into felsic, mafelsic and mafic i.e. acid, intermediate, basic and ultrabasic /ultramafic defined by the type of minerals present. b) Crystallinity: Crystallinity is the ratio between crystallized to non-crystallized materials (glass). They are the following types: Holocrystalline: When rock is mainly composed of, crystals only (well developed crystals). It therefore, known as holocrystalline Nepheline syenite Hemicrystalline: holocrystalline, sub-hedral. granite When rock is composed of both crystals and glass it is called hemicrystalline. Sometimes it is also called mesocrystalline. 2
hemicrystalline Holohyaline When rock are formed of completely or entirely by glass. It is called holohyaline. Glass Significance Plutonic rocks are holocrystalline; hypabyssal rocks are mostly holocrystalline, whereas many volcanic rocks are hemicrystalline or even holohyaline. Rapid cooling and high magma viscosity favours glass formation, where as slow cooling and less magma viscosity promotes crystal growth. Colour and granularity above refer exclusively for hand samples; while crystallinity refers to both hand samples and thin sections (if hemi or holohyaline), crystal shape refers to both hand samples and thin sections where as texture refers mainly to thin sections. Even holohyaline rocks may contain minute embryonic crystals. There are two types: Crystallites- Embryonic crystals that cannot be resolved under the microscope. Microlites - Slightly larger crystals whose optical properties can be identified. Their shape classifies these minute crystals: Globulites, Longulites, Scopulites, Margarites, Trichites and Microlites. Glass is a super-cooled viscous solution with no regular molecular arrangement. It is therefore unstable and tends to form fibrous crystals with the passage of time. This referred to as devitrification. The formation of crystals reduces the volume and leads to the formation of cracks. These are generally curved or concentric (resulting in onion -like shells enclosing small pearl like spheres of glass). This is known as perlitic structure. 3
(c) Granularity Crystals range in size from microlites to meters. However, the general classification is based on visibility with the naked eye (or x10 hand lens). These are mainly two types: Aphanitic: When individual crystals cannot be identified by naked eyes. Aphanitic Texture: The small crystals that cannot see by the eyes or with the help of hand lens know as Aphanitic texture. The entire rock made up of small crystals, which are generally less than 1/2 mm in size. This type of texture usually found in volcanic or hypabyssal (shallow subsurface) environments from by rapid cooling. aphanitic texture (thin section) aphanitic (hand sample) Aphanitic rocks are characterized by textures in which the mineral grains are not visible, while mostly aphanitic rocks may few phenocrysts (smaller or larger grains). This often causes or confuse the student to assume a phaneritic texture, but with a little practice you can quickly distinguish between aphanitic and phaneritic textures. The dark rocks like gabrro are more problematic. Aphanitic rocks may be holohyaline, hemicrystalline or microcrystalline (i.e. individual crystals can be distinguished with the aid of a microscope). They can be cryptocrystalline (individual crystals cannot be resolved under microscope, yet the reaction to polarized light indicates their presence). Phanerocrystalline or Phaneric: When we can identify and distinguish the individual crystals by naked eyes. Examples of Phaneritic Rocks (the two images below show a hand sample and a thin section of phaneritic textured rocks) Phaneritic texture rocks are composed of large crystals that are clearly visible to the eye with or without a hand lens or under microscope. The entire rock is made up of large crystals, which are generally 1/2 mm to several centimeters in size; no fine matrix material is present. This texture forms by slow cooling of magma during the plutonic environment. 4
Phanerocrystalline rocks can be further subdivided on the basis of crystals size: Coarse grained : When average crystal diameter is >5mm. Medium grained : Average crystal diameter vary from 1-5mm Fine grained : When average crystal diameter is <1mm. Coarse grained Medium grained Fine grained (d) Crystal Shape The overall fabric depends largely on crystal shape. This can be defined in two ways (i) by reference to development of crystals faces and (ii) by referring relative crystals dimensions to the three directions in space. i) Crystal faces: The crystal faces may be grouped or classified in to following types Euhedral: When crystal are completely bounded by crystal faces is know as euhedral crystal. Subhedral: when some faces partially developed, but not all the faces is know as subhedral. Anhedral: when crystal faces are totally absent, often dues to other crystals interfering with their growth. ii) Direction in space: It depends upon the type of the mineral present. Equidimensional When crystal more or less equally developed in every direction it is known as equidimensional e.g. Leucite 5
Tabular When crystal developed in two spatial directions is known as tabular e.g. feldspar Prismatic When crystal best developed in one direction is known as prismatic e.g. hornblende There are a number of special names that can be applied to prismatic crystals Acicular if needle-like (e.g. asbestors minerals), Filiform if wire-like fibres (e.g. sillimanite), Fibrous when a number of acicular crystals are grouped together and Stellate when fibrous group radiate from a common center. 6