Consider the history of a single crystal of quartz. Describe how could this quartz grain could (a) form in a granite, (b) become incorporated into a sandstone, and (c) be transformed into a quartzite?

Question

Explain the processes which act on the quartz crystal and the transformations it experiences. Detail the processes which would take place along each portion of this journey.
What will be an ideal response?

Answer 1

Answer: The quartz crystal would originally form from magma, cooling deep in Earth's interior. Because it's part of an intrusion of magma, it cools slowly, so there's plenty of time to grow a nice coarse quartz crystal. It forms last among the minerals in the crystallizing magma, as predicted in Bowen's reaction series. Because of the felsic composition and the coarse grain size, the rock that this quartz (mineral) grain is part of would be referred to as a granite. Once solidified, time goes by, and the granite is uplifted towards the surface. When it gets to the surface, it is attacked by chemical and mechanical weathering. Sheeting occurs, and carbonic acid in rainwater helps to break down the quartz's neighboring feldspars. The few dark silicate minerals in the granite are oxidized. One day, in a big storm, the quartz crystal breaks free and is tumbled down a small stream along with a bunch of clay minerals. The small stream feeds into a big river, and the quartz grain eventually is transported to the ocean. There, it is deposited on a beach full of similar sand-sized grains of quartz. Over geologic time, these sand layers are buried and subjected to confining pressure, and they become lithified to form a sandstone. A new intrusion of magma occurs nearby, and the sandstone is "cooked" by its heat. The small quartz grains recrystallize, fusing together and forming large, equidimensional grains that are randomly oriented. It is now a quartzite, a metamorphic rock.