Quartz is one of the most widespread minerals on Earth, forming a component of countless rock types and soils. This mineral, a form of silicon dioxide (\(\text{SiO}_2\)), is recognized for its crystalline structure and relative abundance. Understanding its physical properties, particularly its hardness, is important for appreciating its role in both nature and industry. Mineral hardness is determined by assessing its resistance to being scratched or abraded by another material.
Understanding Mineral Hardness
Mineral hardness is measured using the standardized Mohs Scale of Mineral Hardness. This ordinal ranking runs from 1 (softest) to 10 (hardest). German mineralogist Friedrich Mohs developed this scale in 1812 using ten common minerals as reference points. The scale works on a simple principle: a material can be scratched by any material with a higher Mohs number, but it will scratch any material with a lower number.
The Mohs scale is relative, meaning the intervals between the numbers are not equal in absolute hardness. For example, the difference in hardness between 9 (corundum) and 10 (diamond) is much greater than the difference between 1 (talc) and 2 (gypsum). This scratch test provides a quick and effective way to assess a material’s potential durability. The hardness value is a direct measure of the strength of the chemical bonds holding the mineral’s atoms together.
The Mohs Ranking of Quartz
Quartz sits firmly at a 7 on the Mohs Scale of Hardness, a placement that defines its general resilience. This number signifies that quartz is resistant to physical abrasion compared to many other substances. In the environment, the vast majority of dust and airborne particles are softer than a Mohs 7. This resistance to common abrasive particles is a major factor in the mineral’s widespread survival.
The \(\text{SiO}_2\) composition creates a robust crystalline structure that resists the breaking of chemical bonds under typical scratching forces. Because of this high ranking, quartz can only be scratched by minerals that are harder than it, such as topaz (8), corundum (9), and diamond (10). This inherent scratch resistance makes quartz a reliable material for both natural geological formations and manufactured products.
Practical Durability and Uses
The Mohs 7 rating translates into a high degree of practical durability, making quartz useful across several industries. Since quartz is harder than most common construction materials, it is widely utilized as an abrasive agent. Quartz sand is processed and used in industrial applications such as sandblasting, grinding media, and as grit for sanding and sawing. Its ability to scratch softer materials allows it to shape and smooth metals, glass, and other stones effectively.
In home construction, this hardness is the primary reason for the popularity of engineered quartz countertops. These surfaces are made from crushed quartz crystals mixed with resins, inheriting the mineral’s scratch-resistant property. The resulting surface is highly resistant to daily wear and tear, including scratches from cookware or utensils, which are generally made of softer metals. Furthermore, quartz is chemically inert with most substances, making it resistant to corrosion from common household cleaners and acids.
Comparing Quartz to Everyday Materials
Placing quartz’s Mohs 7 value into context requires comparing it to materials encountered daily. A human fingernail has a Mohs hardness of approximately 2.5, and a common copper penny registers around 3 to 3.5. Even a standard steel pocket knife or a piece of window glass typically falls between 5.5 and 6.5.
Quartz can easily scratch window glass and steel tools, demonstrating its superior hardness compared to these common household items. This is why beach sand, which is largely composed of quartz, can often etch glass if it is dragged across the surface. Conversely, materials like hardened tool steel or specialized industrial ceramics, which can reach Mohs values of 8 or higher, are able to scratch quartz.