In a rock calcium carbonate predominates. State whether it is :
Try it first: pick an option, then read the explanation.
Understanding Chemical Classification of Rocks
This question focuses on the chemical classification of rocks, which groups them based on their main chemical component. This is distinct from geological classification (how they form) or structural classification (their physical structure).
Detailed Analysis of the Options
C. Calcareous Rock
This is the correct answer. The term "Calcareous" is derived from 'Calx', the Latin word for lime. Calcareous rocks are those in which the primary chemical constituent is calcium carbonate (CaCO₃), also known as lime or calcite.
- Key Component: Calcium Carbonate (Lime).
- Characteristics: These rocks are typically of sedimentary origin and will react with (fizz when in contact with) acids.
- Examples: Limestone, Marble (which is metamorphosed limestone), and Dolomite.
A. Silicious Rock
This classification is for rocks where silica (SiO₂) is the predominant mineral. These rocks are generally very hard and resistant to weathering.
- Key Component: Silica (Quartz).
- Examples: Granite, Sandstone, Quartzite.
B. Argillaceous Rock
This classification applies to rocks in which clay minerals (composed mainly of alumina and silica) are the main component.
- Key Component: Argil (Clay) / Alumina.
- Examples: Shale, Slate, Kaolin.
D. None of these
This is incorrect, as the definition in the question perfectly matches that of a calcareous rock.
Quick Comparison of Chemical Classifications
| Chemical Classification | Predominant Mineral | Key Characteristic | Examples |
|---|---|---|---|
| Calcareous | Calcium Carbonate (Lime) | Reacts with acid | Limestone, Marble |
| Silicious | Silica (Quartz) | Hard and durable | Granite, Sandstone |
| Argillaceous | Argil (Clay) / Alumina | Soft, often plastic | Shale, Slate, Kaolin |
Study Tips
- The Clue is in the Question: The question asks about Calcium Carbonate. The answer is Calcareous. The first four letters match perfectly!
- Know the "-aceous" words: This is the key to chemical classification.
- Calcareous = Calcium
- Silicious = Silica
- Argillaceous = Clay
- Think of an Acid Test: A common field test for limestone is dropping a small amount of weak acid on it. If it fizzes, it's calcareous.