Which characteristic best describes the structure of feldspar?

Feldspar is best described as having a crystalline structure. This characteristic is significant because feldspar is a group of tectosilicate minerals that forms an essential component of many igneous and metamorphic rocks. In a crystalline structure, the atoms are arranged in an orderly repeating pattern, which influences the mineral's physical properties, including hardness and cleavage.

The crystalline nature of feldspar not only affects its aesthetics and optical properties, it also plays a crucial role in how it interacts with other minerals during rock formation. The alignment of the atoms in a crystal lattice facilitates the formation of distinct cleavage planes, which can be observed during mineral identification. This crystalline structure is fundamental to understanding the behavior of feldspar in geological processes.

In contrast, while feldspar may exhibit other characteristics such as granularity or a massive appearance in some contexts, these descriptors do not capture the essential orderly arrangement of atoms that defines its crystalline nature. Thus, the crystalline structure is a defining characteristic that sets feldspar apart from many other forms of minerals.