Why Does Quartzite Tend to Be Very Hard & Resistant?

Quartzite is resistant to breakage.

Quartzite is a common rock made from metamorphosed sandstone and a close cousin of the more commonly recognized quartz. Heat and pressure compress the sandstone with silica until new molecular bonds form that make it far denser that sandstone.

Big Forces

Intense pressure and heat over thousands of years press materials together. While under pressure and occasionally molecularly destabilized by heat, new molecular bonds are created that consolidate quartzite without creating "foliations," or "cleavage" lines in the stone that are vulnerable to fracture. This lack of cleavage makes quartz very resistant to sheering or splitting. That same intense compression results in increased density, resulting in its imperveiousness to moisture and its increased weight per volume.

Intergrowth

The bonds created by sandstone metamorphosis are said to be "inter-grown." They are not merely mechanical locks between materials, but new molecular bonds that "weld" the silica and sandstone into a new material. This is called metamorphosis. These glass-like bonds give quartzite its "scratch-resistant" hardness. These molecular bonds are so "tight" that quartzite resists breaking as well as scratching.

Mohs

Geologists actually employ a scale to describe hardness: the Mohs Hardness Scale. Mohs scale measures hardness by observing resistance to abrasion. Quartzite is around Mohs-7. Diamonds, the hardest rock, are Mohs-9.

References

• The Image: Quartzite Metamorphosis
• Mineral Information Institute: Quartzite
• The Image: Quartzite Metamorphosis

Resources

• Stone Network: Quartzite Hardness