The brittle metalloid element that occurs in sand, quartz, granite, feldspar, and clay is silicon (Si).

Here's why:

* Sand: Primarily composed of silicon dioxide (SiO2), also known as silica.

* Quartz: A crystalline form of silicon dioxide (SiO2).

* Granite: A rock containing quartz, feldspar, and mica, all of which contain silicon.

* Feldspar: A group of minerals containing silicon, aluminum, and oxygen.

* Clay: Formed from the weathering of other silicate minerals (containing silicon and oxygen).

Silicon is a metalloid, meaning it has properties of both metals and nonmetals. It's brittle, like a nonmetal, but can also conduct electricity under certain conditions, like a metal. It's also a key component in the Earth's crust and plays a vital role in many industries, including electronics, construction, and glassmaking.