Difficulty: Easy
Correct Answer: Calcareous rock
Explanation:
Introduction / Context:
Marble is a very common building and decorative stone that students often encounter in daily life and in basic geology. Examinations frequently ask about both the origin of marble and its chemical nature. Geologists classify rocks in several ways, including by how they form and by their chemical composition. This question focuses specifically on the chemical classification of marble, which is very important for understanding its properties, uses, and how it reacts with acids and weathering processes.
Given Data / Assumptions:
Concept / Approach:
Calcareous rocks are rich in calcium carbonate, mainly in the form of the mineral calcite. Siliceous rocks are rich in silica, usually quartz, and argillaceous rocks are rich in clay minerals derived from very fine sediments. Marble forms when limestone, which is already a calcareous sedimentary rock, undergoes metamorphism under heat and pressure. During metamorphism, the calcite crystals recrystallize and grow, producing the interlocking crystalline structure of marble. Because both limestone and marble are dominated by calcium carbonate, marble remains chemically calcareous even though it is texturally a metamorphic rock by origin.
Step-by-Step Solution:
1. Start with the parent rock of marble, which is limestone, a rock composed mainly of calcium carbonate.2. Recognize that during metamorphism, limestone is subjected to heat and pressure, but its chemical composition remains largely the same, dominated by calcite.3. As a result, the rock turns into marble, which shows recrystallized calcite grains but still has a calcareous composition.4. Calcareous rocks are defined as rocks rich in calcium carbonate, and marble fits this definition very well.5. Siliceous rocks, by contrast, are dominated by silica and quartz, which is not the case for marble.6. Argillaceous rocks are dominated by clay minerals and fine particles, which again does not match the composition of marble.7. Therefore, when the question asks for the chemical classification of marble, the correct answer is that marble is calcareous.
Verification / Alternative check:
A simple field or laboratory check supports this classification. When dilute hydrochloric acid is dropped on marble, it effervesces strongly due to the reaction of acid with calcium carbonate, which releases carbon dioxide gas. This is a classic test for calcareous rocks such as limestone and marble. Siliceous rocks, like quartz sandstone, do not show this strong reaction. In addition, when marble is dissolved or weathered, it releases calcium and carbonate ions, which also indicates a calcareous nature. These observations confirm that marble is chemically a calcareous rock, even though it is metamorphic in terms of origin.
Why Other Options Are Wrong:
Option B, siliceous rock, is incorrect because marble is not dominated by silica; it is dominated by calcium carbonate. Option C, argillaceous rock, is wrong because marble does not consist mainly of clay minerals or fine mud particles. Option D, metamorphic rock of mixed composition, describes the origin of marble but does not answer the chemical classification asked in the question. The question is specifically about chemical nature, and only calcareous rock accurately expresses the dominant composition of marble.
Common Pitfalls:
Students often confuse genetic classification with chemical classification. They know that marble is a metamorphic rock and may be tempted to pick the metamorphic related option even when the question clearly asks for chemical classification. Another pitfall is thinking that any hard decorative stone must be siliceous, which is not always true. Paying close attention to whether the exam asks about composition, structure, or origin helps to avoid such mistakes.
Final Answer:
Chemically, marble is best classified as a calcareous rock because it is mainly composed of calcium carbonate, usually in the form of calcite.
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