Question

The combinations listed below represent major minerals observed in four igneous rocks: 
(i) Olivine and Anorthite, (ii) K-feldspar and Quartz, (iii) Mg-Ca-pyroxene and Ca-Na-plagioclase, (iv) Amphibole and Na-Ca-plagioclase 
Arrange these mineral combinations based on decreasing temperature of magma crystallization.

• A. \((i)>(ii)>(iii)>(iv)\)
• B. \((i)>(iii)>(iv)>(ii)\)
• C. \((i)>(iv)>(iii)>(ii)\)
• D. \((ii)>(i)>(iv)>(iii)\)

Hint:

In igneous petrology, the order of crystallization follows the Bowen’s reaction series, where minerals like olivine crystallize at the highest temperatures and minerals like quartz crystallize at the lowest.

Answer 1

The Correct Option is B

Step 1: Understanding crystallization temperatures in igneous rocks.
The order in which minerals crystallize depends on their melting points, with minerals crystallizing at higher temperatures first.

- Olivine and Anorthite: These minerals crystallize at the highest temperatures, making combination (i) the first in the sequence.
- Mg-Ca-pyroxene and Ca-Na-plagioclase: These minerals crystallize at moderate temperatures and come after (i) in the sequence.
- Amphibole and Na-Ca-plagioclase: These minerals crystallize at lower temperatures and come after (ii) in the sequence.
- K-feldspar and Quartz: These minerals crystallize at the lowest temperatures, making combination (ii) last in the sequence.

Step 2: Arranging the mineral combinations.
Based on their crystallization temperatures, the correct order is:

(i) Olivine and Anorthite > (iii) Mg-Ca-pyroxene and Ca-Na-plagioclase > (iv) Amphibole and Na-Ca-plagioclase > (ii) K-feldspar and Quartz.