Question

Which one among the following mineral pairs crystallise early during the cooling of a basaltic melt?

• A. Forsterite and albite
• B. Biotite and anorthite
• C. Enstatite and bytownite
• D. Forsterite and quartz

Answer 1

The Correct Option is C

To answer the question about which mineral pairs crystallize early during the cooling of a basaltic melt, we need to consider the crystallization sequence of minerals from a basaltic melt, which is closely related to Bowen's Reaction Series. 

Bowen's Reaction Series explains the order of mineral crystallization from magma. During the cooling of a basaltic melt, the mafic minerals, which are rich in magnesium and iron, tend to crystallize earliest. These include olivine (forsterite) and pyroxenes (such as enstatite).

Additionally, plagioclase feldspar minerals also begin crystallizing early in the sequence. The plagioclase feldspars range from calcium-rich (anorthite) to sodium-rich (albite) compositions as cooling progresses.

By analyzing the options, we can eliminate:

• Forsterite and albite: Forsterite (an olivine) crystallizes early, but albite (a sodium-rich feldspar) forms much later as it is a product of lower-temperature conditions.
• Biotite and anorthite: While anorthite (a calcium-rich plagioclase) forms early, biotite does not crystallize early in basaltic sequences; it forms at relatively lower temperatures.
• Forsterite and quartz: Forsterite crystallizes early, but quartz is one of the last minerals to crystallize during cooling.

 

Thus, the correct answer is Enstatite and bytownite:

• Enstatite is a pyroxene mineral, and together with others like forsterite, it forms early.
• Bytownite is a calcium-rich plagioclase and forms early in basaltic melts.

This aligns with the typical crystallization sequence for basalts, where early-crystallizing minerals are those richer in iron, magnesium, and calcium.