Describe how one can compositionally differentiate between a marble and quartzite.
The correct answer and explanation is :
Marble and quartzite are both metamorphic rocks, but they differ in composition, texture, and formation, which can help in their compositional differentiation.
Marble Composition:
Marble is primarily composed of calcite (CaCO₃), which is the crystalline form of calcium carbonate. It forms when limestone undergoes metamorphism, a process in which heat and pressure cause the limestone to recrystallize into a denser rock. The primary mineral in marble is calcite, though it may contain varying amounts of other minerals such as mica, graphite, or clay, depending on the impurities present in the original limestone.
Quartzite Composition:
Quartzite, on the other hand, is composed mainly of quartz (SiO₂). It forms from the metamorphism of quartz-rich sandstone. When sandstone undergoes high heat and pressure, the quartz grains fuse together, making quartzite a very hard and durable rock. Quartzite may also contain small amounts of feldspar, mica, or other minerals, but quartz is by far the most abundant component.
Key Compositional Differences:
- Primary Mineral Content:
- Marble is made primarily of calcite (CaCO₃).
- Quartzite is composed predominantly of quartz (SiO₂).
- Reaction to Acid:
- Marble reacts with dilute hydrochloric acid (HCl), which causes effervescence due to the release of carbon dioxide from the calcite.
- Quartzite does not react with HCl because quartz is not soluble in acid.
- Hardness:
- Quartzite is much harder than marble. On the Mohs scale, quartzite typically measures around 7, while marble is much softer, measuring about 3-4 due to the calcite content.
- Appearance:
- Marble tends to have a fine-grained, smooth texture and often displays veining from impurities like graphite or mica.
- Quartzite is typically more granular and may have a more textured appearance, often with a glassy luster due to the recrystallization of quartz.
These differences in composition and physical properties help in distinguishing marble from quartzite.