Sand Composition of Sandstones and Pebbly Sandstones 5XX 7XX
Sandstones are composed mainly of the mineral quartz, along with different amounts of other constituents. Variations in quartz content affect a rock’s color and hardness. Rocks with greater than 90 percent quartz grains are almost white in color, and are extremely hard and brittle when struck with a hammer. Many quartz-rich sandstones also have quartz cement in the pore spaces between grains, which contributes to the hardness. When quartz-rich sandstones are broken, examination of the butt end of the core often reveals a sugary texture that results from fracturing through grains, rather than around grains.
Darker-colored sandstones may contain additional minerals such as feldspar, fragments of other kinds of rock, and clay minerals. The other minerals contribute to their darker color and relative softness. These lithic sandstones typically have quartz contents between 40 and 60 percent.
If the sandstone has visible quartz grains but effervesces when HCL is applied directly or to a powder, go to the Carbonate Cements page for further description.
There is a continuum between end-member lithic and quartzose varieties of sandstones. There is a middle group that is darker than quartz-rich specimens, but just as hard. Drillers often refer to these as “hard sandstones.” These hard sandstones likely have quartz contents between 60 and 90 percent.
The three classes of sandstone and pebbly sandstone in the Ferm classification based on mineral composition or proxies for mineral composition are:
- 55X 75X Quartzose
- 56X 76X Hard
- 54X 74X Gray (lithic)
Pebbly sandstones are also characterized by the types of pebbles that are present. To determine the last digit of the rock code for this group, go to the Pebble Composition page.
Sandstones without pebbles are further categorized according to the alignment of the sand grains and other constituents recorded in the third and decimal digit of the code. Go to the Bedding page to further classify this group after sand composition has been determined.