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Question

Match the following igneous textures in Group-I with their definitions in Group-II.

Group-I

Group-II

PVitrophyre1Alkali feldspar rimmed by plagioclase
QRapakivi2Aggregate of radially arrayed, needle-like crystals of plagioclase with or without clinopyroxene
ROcelli3Sub-parallel skeletal, platy olivine and/or pyroxene
SSpinifex4Large phenocrysts within a glassy matrix

The correct answer is
P-4; Q-1; R-2; S-3

Matching Igneous Textures and Definitions

The question requires matching specific igneous textures listed in Group I with their definitions in Group II.

Texture-Definition Matches

  • P - Vitrophyre: This texture is characterized by large phenocrysts dispersed within a fine-grained or glassy matrix. This matches definition 4: "Large phenocrysts within a glassy matrix".
  • Q - Rapakivi: This specific texture involves alkali feldspar crystals typically mantled by a later rim of plagioclase. This corresponds to definition 1: "Alkali feldspar rimmed by plagioclase".
  • R - Ocelli: Ocelli (singular: ocellus) are typically small, rounded, or irregular masses, sometimes described as eye-like. Definition 2, "Aggregate of radially arrayed, needle-like crystals of plagioclase with or without clinopyroxene", best describes a possible form or component related to ocelli formation in certain contexts, especially when considering the available options.
  • S - Spinifex: This texture is distinctive, composed of long, skeletal, needle-like crystals, often olivine or pyroxene, arranged in a felted or sub-parallel manner. This matches definition 3: "Sub-parallel skeletal, platy olivine and/or pyroxene".

Correct Pairing

Based on the analysis:

  • P matches with 4
  • Q matches with 1
  • R matches with 2
  • S matches with 3

Therefore, the correct option is P-4; Q-1; R-2; S-3.

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Important Questions from Igneous Petrology (Classification, Forms, Textures, Genesis)

  1. The given table shows modal volume percentage of quartz, alkali feldspar and plagioclase in coarse-grained rocks, X and Y. Which of the following rock names of X and Y are CORRECT according to the IUGS classification?

    MineralX (volume %)Y (volume %)
    Quartz1525
    Plagioclase2060
    Alkali feldspar6515
  2. Which one of the following is a mafic volcanic rock?
  3. Match the rock types in Group-I with their most likely corresponding lithospheric / tectonic settings of formation in Group-II
    Group-IGroup-II
    P. Boninite1. Continental anorogenic
    Q. Lamproite2. Island-arc
    R. Phonolite3. Continental collision
    S. Leucogranite4. Intraplate oceanic
  4. The mole percentages of SiO$_2$, Al$_2$O$_3$ and K$_2$O in a granitic rock are 84.21, 7.89 and 7.89, respectively. The molar proportion (in %) of K-feldspar in the rock is ________ [ round off to one decimal place]
  5. Which one of the following statements about igneous rocks is CORRECT?
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