Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years. Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity. Calcium carbonate is one of the most useful and versatile materials known to man.
Many of us encounter calcium carbonate for the first time in the school classroom, where we use blackboard chalk. Chalk has been used as a writing tool for over 10,000 years and is a fine, microcrystalline material. As limestone, calcium carbonate is a biogenic rock, and is more compacted than chalk. As marble, calcium carbonate is a coarse-crystalline, metamorphic rock, which is formed when chalk or limestone is recrystallised under conditions of high temperature and pressure. Large deposits of marble are found in North America and in Europe; e.g., in Carrara, Italy, the home of the pure white "statuario" from which Michelangelo created his sculptures.
Calcium carbonate, as it is used for industrial purposes, is extracted by mining or quarrying. Pure calcium carbonate can be produced from marble, or it can be prepared by passing carbon dioxide into a solution of calcium hydroxide. In the latter case calcium carbonate is derived from the mixture, forming a grade of product called “precipitated calcium carbonate,” or PCC. PCC has a very fine and controlled particle size, on the order of 2 microns in diameter, particularly useful in production of paper. The other primary type of industrial product is “ground calcium carbonate,” or GCC. GCC, as the name implies, involves crushing and processing limestone to create a powdery-like form graded by size and other properties for many different industrial and pharmaceutical applications.
Quartz is the most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz.
A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad catagories. The specimens could be separated by answers to the following questions: color?, shade?, pyramidal?, prismatic?, druzy?, twinned?, sceptered?, phantomed?, included?, tapered?, coated?, microcrystalline?, stalactitic?, concretionary?, geoidal?, banded?, etc. Multiple combinations of these could produce hundreds of unique possibilities.
- Chemistry: SiO2, Silicon dioxide
- Class: Silicates
- Subclass: Tectosilicates
- Group: Quartz
- Uses: silica for glass, electrical components, optical lenses, abrasives, gemstones, ornamental stone, building stone, etc.
- The Physical Properties of Quartz.
Quartz is not the only mineral composed of SiO2. There are no less than eight other known structures that are composed of SiO2. These other substances and quartz are polymorphs of silicon dioxide and belong to an informal group called the Quartz Group or Silica Group. All members of this group, except quartz, are uncommon to extremely rare on the surface of the earth and are stable only under high temperatures and high pressures or both. These minerals have their own unique structures although they share the same chemistry, hence the term polymorph, which means many forms.