Pearl is a hard object produced within the soft tissues of a mollusk. Which one of the following is the main constituent of pearl?
Calcium carbonate
Pearls are fascinating natural treasures formed within the soft tissues of certain mollusks, such as oysters and mussels. They are created layer by layer as the mollusk secretes a substance called nacre, also known as mother-of-pearl, around an irritant inside its shell.
The question asks about the main constituent of a pearl. Let's look at the options provided and determine which substance makes up the largest part of a pearl's composition.
Pearls are primarily composed of calcium carbonate in a crystalline form. This calcium carbonate is laid down in tiny, overlapping layers. The specific crystalline form found in pearls is mainly aragonite, although calcite can also be present. These crystals are cemented together by a protein known as conchiolin. The arrangement of these layers and the interaction of light with them give pearls their characteristic luster and iridescence.
Based on the chemical composition of natural and cultured pearls, calcium carbonate is overwhelmingly the main constituent.
| Substance | Chemical Formula | Main Role in Pearl? |
|---|---|---|
| Calcium carbonate | \(\text{CaCO}_3\) | Yes (Main constituent) |
| Calcium oxide | \(\text{CaO}\) | No |
| Calcium nitrate | \(\text{Ca(NO}_3)_2\) | No |
| Calcium sulphate | \(\text{CaSO}_4\) | No |
Therefore, the substance that is the main constituent of pearl is calcium carbonate.
| Component | Approximate Percentage | Role |
|---|---|---|
| Calcium Carbonate (\(\text{CaCO}_3\)) | 90-95% | Forms the crystalline structure (nacre/aragonite) |
| Organic Matrix (Conchiolin) | 4-8% | Acts as the 'glue' binding crystals; provides framework |
| Water | Balance (typically < 4%) | Present in the organic matrix |
The formation of a pearl is a defense mechanism for the mollusk. When an irritant, such as a parasite or a piece of shell, enters the mollusk, the animal secretes layers of nacre around it to protect itself. Over time, these layers build up to form a pearl.
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