Calcium Carbonate (CaCO?)

Calcium carbonate is a naturally occurring inorganic compound with the chemical formula CaCO?. It is one of the most common minerals on Earth, widely found in limestone, marble, shells, pearls, eggshells, etc.  

1. Basic Properties 

- Chemical formula: CaCO?  

- Molar mass: 100.09 g/mol  

- Appearance: White powder or odorless crystals (different crystal forms)  

- Solubility:  

- Almost insoluble in water (solubility ~0.0014 g/100 mL, 25°C)  

- Soluble in acids, releasing CO?: 

- Thermal stability: Decomposes at 825°C into calcium oxide (CaO) and carbon dioxide (CO?):  

2. Main Crystal Forms (Polymorphs)

Calcium carbonate has three common crystal structures, which affect its physical properties and applications:  

- Calcite

   - Most stable, hexagonal crystal system, used in construction materials and fillers.  

- Aragonite

   - Orthorhombic crystal system, biologically sourced (e.g., shells, corals), used in high-end fillers and biomaterials.  

- Vaterite

   - Least stable, metastable, used in drug sustained-release and nanomaterials.  

3. Main Sources 

- Natural minerals:  

  - Limestone (main component CaCO?)  

  - Marble (metamorphosed limestone, high purity)  

  - Chalk (formed by microbial deposition)  

- Biological sources:  

  - Shells, eggshells, corals (mainly composed of aragonite-type CaCO?).  

- Industrial synthesis:  

  - Precipitation method: Bubbling CO? into calcium chloride (CaCl?) solution or adding sodium carbonate (Na?CO?):  

4. Main Applications

(1) Industry and Construction Materials

- Cement & Lime: Calcining CaCO? to produce quicklime (CaO), used in construction and steel industries.  

- Fillers & Coatings:  

  - Papermaking (improves whiteness and smoothness)  

  - Plastics, rubber (enhances mechanical properties)  

  - Paints (thickening, wear resistance)  

(2) Food and Pharmaceuticals

- Food additive (E170):  

  - Anti-caking agent (e.g., in table salt, milk powder)  

 - Calcium supplement (must meet food-grade standards)  

- Pharmaceutical excipient:  

  - Antacid (neutralizes stomach acid)  

  - Tablet filler  

(3) Agriculture and Environment

- Soil conditioner: Neutralizes acidic soil (raises pH).  

- Water treatment: Removes heavy metals (e.g., Pb2?) and acidic pollutants from water.  

(4) Other Applications

- Toothpaste abrasive (mild grinding, cleans teeth).  

- Cosmetics (filler in foundation, powder).  

Laboratory: Preparation of CO?, calcium standard solutions.  

5. Safety and Precautions 

- Toxicity: Low toxicity, but long-term inhalation of dust may cause pneumoconiosis (industrial protection must meet standards).  

- Food and pharmaceutical grade: Requires high purity (free from heavy metals, arsenic, and other impurities).  

- Storage: Dry environment, avoid contact with acids.