Understanding the Mohs Hardness Scale
Before diving into what is the 7th mineral, it's essential to understand the Mohs hardness scale itself. Developed in 1812 by German geologist Friedrich Mohs, the scale is a qualitative measure of scratch resistance, from softest (1) to hardest (10). It uses ten reference minerals, each capable of scratching the minerals with a lower ranking. While simple, it remains a crucial and widely used method for field mineral identification due to its practical application. For instance, a mineral with a hardness of 7 can scratch a mineral with a hardness of 6, but not a mineral with a hardness of 8.
The Identity of the 7th Mineral: Quartz
The 7th mineral on the Mohs scale is quartz, a mineral with a chemical formula of SiO₂ (silicon dioxide). Its hardness of 7 means it can scratch glass, which has a typical hardness of 5.5 to 6.5, but can, in turn, be scratched by harder minerals like topaz (8), corundum (9), and diamond (10). Quartz is a fundamental component of the Earth's crust, second only to feldspar in abundance. Its prevalence is due to silicon and oxygen being the two most common elements in the crust.
Diverse Varieties and Characteristics of Quartz
Quartz is not just one simple mineral but a family of varieties, each with its own unique appearance and properties, despite all having a hardness of 7. These variations arise from different formation conditions or trace impurities.
- Clear Quartz (Rock Crystal): The colorless and transparent form, prized for its clarity.
- Amethyst: A purple variety, with its color caused by irradiation and iron impurities.
- Citrine: Ranging from pale yellow to brownish orange, the color is due to traces of ferric iron.
- Rose Quartz: A pink variety, the delicate color is often caused by tiny inclusions of a fibrous mineral.
- Smoky Quartz: A grey to brown, translucent variety, with its color also resulting from natural irradiation.
Quartz exhibits a hexagonal crystal system and often forms six-sided prisms with pointed terminations. It is also known for its conchoidal fracture, a characteristic break with smooth, curved surfaces, and a glassy, or vitreous, luster.
The Geological Importance of Quartz
As a primary rock-forming mineral, quartz plays a significant role in geology. It is a major constituent of many common rocks, including granite, sandstone, and gneiss. Because of its hardness and chemical durability, quartz is highly resistant to weathering, making it a persistent component of sediments and sand on beaches and in deserts worldwide. Its piezoelectric properties—the ability to generate an electric charge when subjected to mechanical stress—make it valuable in electronics for watches, radios, and computers.
Comparing Mohs Scale Minerals
| Mohs Hardness | Reference Mineral | Can Scratch | Scratched By |
|---|---|---|---|
| 1 | Talc | None | All others |
| 2 | Gypsum | Talc | All others |
| 3 | Calcite | Gypsum | All others |
| 4 | Fluorite | Calcite | All others |
| 5 | Apatite | Fluorite | All others |
| 6 | Orthoclase Feldspar | Apatite | All others |
| 7 | Quartz | Orthoclase Feldspar | Topaz, Corundum, Diamond |
| 8 | Topaz | Quartz | Corundum, Diamond |
| 9 | Corundum | Topaz | Diamond |
| 10 | Diamond | All others | None |
The Ambiguity of 'The 7th Mineral'
While quartz is the definitive answer on the Mohs hardness scale, the phrase "the 7th mineral" can sometimes refer to other contexts, which is an important point of clarification for geologists. For instance, minerals can also be classified by chemical composition. There are seven major mineral classes, such as silicates, oxides, and carbonates, but this is a classification system, not a numerical ranking. A different interpretation might refer to the abundance of minerals in the Earth's crust, where feldspar is the most common, and quartz is often cited as second. A final possible context involves human nutrition, where one might be referring to one of the seven major macrominerals. However, without any specific qualifiers, the Mohs hardness scale is the most direct and common interpretation.
Conclusion
In summary, when someone asks what is the 7th mineral, the most accurate answer refers to quartz on the Mohs hardness scale. With a hardness of 7, quartz is a robust and resilient mineral that is incredibly common in the Earth's crust. Its wide range of applications—from electronics to construction materials—demonstrates its enduring value beyond its simple ranking. Understanding quartz and its position on the scale provides a useful entry point into the fascinating field of mineralogy and geological science.
For more information on mineral identification, visit the United States Geological Survey website.
