Final answer:
Hornblende has two cleavage planes at 60 and 120 degrees. Cleavage is a key trait in identifying minerals and gives insight into a rock's geological history.
Step-by-step explanation:
Hornblende, which is another name for amphibole, has two cleavage planes at 60 and 120 degrees angles. These cleavage angles are characteristic and help distinguish hornblende from other similar minerals, such as pyroxene, which has cleavage at 90-degree angles. It's essential for geology students and professionals to understand mineral cleavage as it reveals critical insights into the mineral's identity and the geological history of the rocks in which they are found.
When observing mineral cleavage, you may not always notice that the entire mineral sample corresponds to the shape revealed by the cleavage planes. Using a hand lens can help detect slight variations in the surface 'elevation' which indicate cleavage. This examination is crucial in identifying which type of mineral you're examining, as the cleavage is a reliable diagnostic property in mineral identification.