November 14, 2023
A metamorphic rock that consists primarily of interlocking grains of the mineral quartz. Quartzite forms when quartz-rich sandstone undergoes metamorphism.
The term “quartzite” is an example of a geologic term that can be a little confusing. Most of the time, geologists use the term “quartzite” to refer to a metamorphic rock, which is the definition that I have provided above. Sometimes, geologists also use the term “quartzite” to refer to a sandstone that has quartz grains that are very well cemented together, but which has not been transformed in to a metamorphic rock. In my experience as a geologist, this second usage is much less common. I think it was more commonly used in the past. To distinguish between different types of quartzite, geologists can use the terms “metaquartzite” to refer to the metamorphic variety and the term “orthoquartzite” to refer to the sedimentary variety. For the rest of this post, I’ll focus on metaquartzite, which I will just call “quartzite”.
Quartzite is a beautiful metamorphic rock that, at first glance, sometimes looks like sandstone. Quartzite can even retain the original cross-bedding that can be seen in many sandstone rocks, for example as shown in the pictures of quartzite outcrops from Minas Gerais, Brazil, shown in this post.
However, the quartz grains in a quartzite look different to those in a sandstone – they have recrystallized so that that are more or less equal in size. Compared to the original quartz grains that were in the sandstone, quartz grains in quartzite are larger and more polygonal in shape. The grains are tightly interlocked. A characteristic of quartzite is that when it breaks, it will break through grains, not just along grain boundaries.
Here’s a closer view of a quartzite hand sample:
Quartzite is a very hard rock that is highly resistant to erosion. Thus, quartzite can often be found on ridges and on the tops of hills. From a distance, quartzite may look like sandstone – so you may need to look at it close up in order to identify it as quartzite. Quartzite is most commonly white or gray in color, but can also be other colors, such as tan, pink, and yellow.
That’s all for this week’s post – stay tuned for next week!