Answer
If a rock appears to be rich in quartz but effervesces with acid, it indicates that the rock contains a significant amount of calcite (\( \text{CaCO}_3 \)) in addition to the quartz (\( \text{SiO}_2 \)). Calcite is a mineral that reacts with acid, producing carbon dioxide gas (\( \text{CO}_2 \)), which is what causes the effervescence.
Here's a possible explanation for the rock's composition:
1. The rounded glassy particles that appear to be quartz are indeed quartz grains. Quartz is a hard mineral and would easily scratch glass, as you observed during the hardness test.
2. The presence of calcite in the rock is responsible for the fizzing or effervescence when acid is applied. When a drop of acid (usually dilute hydrochloric acid, \( \text{HCl} \)) comes into contact with calcite, a chemical reaction occurs, resulting in the release of carbon dioxide gas:
\[ \text{Calcite (} \text{CaCO}_3 \text{)} + \text{Acid (} \text{HCl} \text{)} \rightarrow \text{Calcium chloride (} \text{CaCl}_2 \text{)} + \text{Carbon dioxide (} \text{CO}_2 \text{)} + \text{Water (} \text{H}_2\text{O} \text{)} \]
The released carbon dioxide gas produces bubbles and causes the fizzing effect.
So, the rock sample is likely a sedimentary rock that consists mainly of rounded quartz grains, but it also contains a significant proportion of calcite (\( \text{CaCO}_3 \)). This combination of minerals is not uncommon in certain sedimentary rocks, and the acid test helps confirm the presence of calcite, despite the rock's dominant appearance of quartz (\( \text{SiO}_2 \)).
Work Step by Step
If a rock appears to be rich in quartz but effervesces with acid, it indicates that the rock contains a significant amount of calcite (\( \text{CaCO}_3 \)) in addition to the quartz (\( \text{SiO}_2 \)). Calcite is a mineral that reacts with acid, producing carbon dioxide gas (\( \text{CO}_2 \)), which is what causes the effervescence.
Here's a possible explanation for the rock's composition:
1. The rounded glassy particles that appear to be quartz are indeed quartz grains. Quartz is a hard mineral and would easily scratch glass, as you observed during the hardness test.
2. The presence of calcite in the rock is responsible for the fizzing or effervescence when acid is applied. When a drop of acid (usually dilute hydrochloric acid, \( \text{HCl} \)) comes into contact with calcite, a chemical reaction occurs, resulting in the release of carbon dioxide gas:
\[ \text{Calcite (} \text{CaCO}_3 \text{)} + \text{Acid (} \text{HCl} \text{)} \rightarrow \text{Calcium chloride (} \text{CaCl}_2 \text{)} + \text{Carbon dioxide (} \text{CO}_2 \text{)} + \text{Water (} \text{H}_2\text{O} \text{)} \]
The released carbon dioxide gas produces bubbles and causes the fizzing effect.
So, the rock sample is likely a sedimentary rock that consists mainly of rounded quartz grains, but it also contains a significant proportion of calcite (\( \text{CaCO}_3 \)). This combination of minerals is not uncommon in certain sedimentary rocks, and the acid test helps confirm the presence of calcite, despite the rock's dominant appearance of quartz (\( \text{SiO}_2 \)).
