siliciclastic sedimentary rocks are composed of mainly silicate particles derived from the weathering of older rocks and pyroclastic volcanism. while grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories: conglomerates, sandstones, and mudrocks. the term "clay" is used to classify particles smaller than .0039 millimeters. however, the term can also be used to refer to a family of sheet silicate minerals. "silt" refers to particles that have a diameter between .062 and .0039 millimeters. the term "mud" is used when clay and silt particles are mixed in the sediment; "mudrock" is the name of the rock created with these sediments. furthermore, particles that reach diameters between .062 and 2 millimeters fall into the category of sand. when sand is cemented together and lithified it becomes known as sandstone. any particle that is larger than two millimeters is considered gravel. this category includes pebbles, cobbles and boulders. like sandstone, when gravels are lithified they are considered conglomerates.