Physical and Chemical Characters of Rock-Forming Sicate Mineral Groups | Geology Optional Notes for UPSC PDF Download

Rock-Forming Minerals

• Rock-forming minerals are the key components of crustal rocks, constituting the bulk of common rocks found on Earth.
• Essential minerals are those that are fundamental to the rock's composition, while accessory minerals are present in smaller quantities.
• Silicate minerals, including quartz and feldspar, are the most prevalent rock-forming minerals, making up a significant portion of the Earth's crust.
• Other common rock-forming minerals include carbonates, oxides, sulfides, and sulfates.

Composition of Silicate Minerals

• Silicate minerals, comprising oxygen and silicon along with other cations, are the most abundant minerals in the Earth's crust.
• They can be classified as non-ferromagnesian silicates (e.g., quartz, muscovite mica, feldspar) and ferromagnesian silicates (e.g., olivine, pyroxene, amphibole).

Common Rock-Forming Silicate Mineral Groups

• Olivine group
• Garnet group
• Mica group
• Feldspar group
• Feldspathoids and Silica group

Significance in Igneous Rocks

• Igneous rocks contain important components such as olivine, pyroxene, amphibole, mica, feldspar, feldspathoid, and quartz among others.

Other Mineral Groups

• Carbonate minerals like calcite and dolomite, along with anhydrite and clay minerals, play crucial roles in rock formation.
• Pyroxene group and Amphibole group are also notable rock-forming mineral groups.

Unit 5: Sedimentary and Metamorphic Rocks

Component of Sedimentary Rocks

• Garnet and other rock-forming minerals like kyanite, andalusite, sillimanite, staurolite, chlorite, serpentine, wollastonite, and glaucophane are essential components of metamorphic rocks.
• Accessory rock-forming minerals include zircon, apatite, magnetite, ilmenite, sphene, fluorite, and a few sulphide minerals. However, for this unit, we will focus on common rock-forming silicate and carbonate minerals only.

Common Rock-Forming Silicate and Carbonate Minerals

• Common silicate minerals to be discussed include quartz, feldspar (orthoclase, microcline, plagioclase), mica (muscovite, biotite), pyroxene (augite, hypersthene), amphibole (hornblende), feldspathoid (nepheline), olivine, and garnet.
• Calcite is the primary rock-forming mineral from the carbonate group, which we will explore further in the subsequent sections.

Classification of Minerals

Olivine Group

• Olivine is a group of common rock-forming minerals primarily found in dark-colored mafic and ultramafic igneous rocks.
• It is a magnesium iron silicate with the chemical composition (XY)2SiO4, where X is Fe and Y is Mg.
• Olivine typically appears green and can have compositions ranging from Mg2SiO4 (Forsterite - Mg rich) to Fe2SiO4 (Fayalite - Fe rich), with varying magnesium and iron ratios.

Garnet Group

• Minerals of the garnet group are commonly used as gemstones and abrasives and have a general chemical composition of X3Y2(SiO4)3, where X can be Ca, Mg, Fe2, or Mn2, and Y can be Al, Fe3, Mn3, V3, or Cr3.
• Garnet subgroup includes pyralspites (pyrope, almandine, spessartine) and ugrandites (uvarovite, grossular, andradite) with distinct characteristics and occurrences in different types of rocks.

Pyroxene Group

• The pyroxene group comprises significant and abundant rock-forming ferromagnesian silicates found in various igneous and metamorphic rocks.
• Pyroxene minerals are prevalent in igneous and metamorphic rocks, with crystallization occurring in different geological settings.

Pyroxene Group of Minerals

• Pyroxene minerals belong to the orthorhombic and monoclinic crystal systems.
• They are similar to amphibole group minerals but lack hydroxyl groups in their structure.
• Due to the absence of hydroxyls, pyroxenes have slightly higher densities.
• The general chemical formula for pyroxene minerals is XYZ₂O₆.
• X can be Na, Ca², Mn², Fe², Mg², or Li; Y can be Mn², Fe², Mg², Fe³, Al³, Cr³, or Ti⁴; and Z is Si⁴ or Al³.
• Augite is the most common pyroxene mineral, typically dark green to black in color.

Amphibole Group of Minerals

• Amphiboles are prevalent in metamorphic and some igneous rocks.
• They are hydrous minerals and can transform into pyroxenes under specific conditions.
• Three main groups of amphiboles exist: Ca-poor, Ca-rich, and alkali amphiboles.
• Hornblende is a common amphibole, characterized by longer, shinier crystals compared to augite.
• Basal sections of amphiboles are six-sided and diamond-shaped.
• The general chemical formula for amphibole minerals is TYAX₂Y₅Z₈O₂₂(OH,F)₂.

Granitic to Gabbroic Transition

• Amphiboles transform into anhydrous minerals, mainly pyroxenes, at high temperatures.

Mica Group

• Micas account for about 4% of the Earth's crust.
• Mica is recognized by its ability to peel into thin, flat sheets or flakes.
• The general formula for mica minerals is X2Y4-6Z8O20(OH,F)4.
• Muscovite is a common white mica, while biotite is dark and shiny.

Feldspar Group

• Feldspar is the most abundant mineral in the Earth's crust.
• It is a light-colored mineral with a porcelain-like appearance.
• Feldspar exists in two subgroups: alkali feldspars and plagioclase feldspars.
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  • Alkali feldspars, such as orthoclase and microcline, are potassium-rich.
  • Plagioclase feldspar forms a solid solution between Na- and Ca-rich end-members.

Feldspathoid Group

• Feldspathoid minerals share similarities with feldspars but form when there is insufficient silica.
• Nepheline, kalsilite, leucite, and sodalite are examples of feldspathoids.

Silica Group

• Minerals in this group have the chemical composition SiO2.
• Quartz, a key mineral in this group, varies in color and is transparent or translucent.

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Mineralogical Concepts

• Quartz Characteristics

  Quartz appears milky white in larger masses and is known for its hardness, being able to scratch a steel knife blade. In rocks, quartz grains are often irregular in shape due to the rarity of crystal faces and the absence of cleavage.

• Carbonate Minerals

  Carbonates are a significant group of minerals commonly found in sedimentary environments, evaporite deposits, and hydrothermal veins. They form when carbon dioxide is available to create the carbonate ion. Calcite and dolomite are prominent examples, with dolomite primarily occurring in rocks that were once limestone formations.

• Common Rock-Forming Mineral Groups

  The table below summarizes the common rock-forming mineral groups along with their idealized formulas and silicate structures:

  • Olivine

    Formula: (Mg,Fe)2 SiO4

    Cleavage Plane: Absent

    Structure: Nesosilicate

  • Garnet

    Formula: (CA, Mg,Fe, or Mn)3(Fe, Mn, V or Cr) ₂(SiO₄)₃

    Cleavage Plane: Absent

    Structure: Nesosilicate

  • Pyroxene (Augite)

    Formula: (Ca, Na) (Mg, Fe, Al) (Si, Al)₂O₆

    Cleavage Plane: 1 at 87° and 93°

    Structure: Inosilicate-double chain

  • Mica

    Formula: (Na or K)(Mg,Fe, Ca, Na)₂(Mg,Fe, Al)₅(Si or Al)₈O₂₂(OH, F)₂

    Cleavage Plane: 1

    Structure: Phylosilicate

  • Amphibole (Hornblende)

    Formula: K(Mg,Fe)₂-₃Al₁-₂Si₈O₂₂-₃(OH,F)₂

    Cleavage Plane: 2 at 87° and 93°

    Structure: Phylosilicate

  • Feldspars

    Formula: KAl₂(AISi₃O₁₀)(OH)₂

    Cleavage Plane: 2 at 90°

    Structure: Tectosilicate

  • And more...

These mineral groups play a crucial role in the formation of rocks and will be further explored in Units 6 and 7, where we will delve into their physical properties.