Final answer:
The mineral from the given list that has three planes of cleavage not at 90 degrees is calcite. This characteristic helps distinguish calcite from other minerals like halite, fluorite, and quartz, which have different cleavage or fracture properties. Option number d is correct.
Step-by-step explanation:
The question asks which mineral has three planes of cleavage that are not at 90 degrees. Among the list provided, the mineral with three planes of cleavage at angles that are not 90 degrees is calcite.
Calcite cleaves in three directions but not at right angles. In contrast, minerals like halite and fluorite have three planes of cleavage at 90 degrees, while quartz does not exhibit cleavage but rather fractures. It's vital for students to understand the concept of cleavage in minerals, as it helps in identifying and differentiating them based on how they break along their planes due to the presence of weak chemical bonds.