Key to Identifying Major Rock Groups
The major rock group is recorded in the first digit of the Ferm code. To get started, test each constituent of the rock with weak (10 percent) hydrochloric acid (HCl).
If all parts of the rock effervesce readily:
Go to the Limestone page for further characterization.
If the rock has folded, steeply inclined, or convoluted bedding:
If the rock has a fine-grained, smooth texture, and:
- Has more than 50 percent of a reddish-brown, dense, hard material that effervesces weakly when HCl is applied to a powder produced by scratching the material with a knife blade, or
- Is buff, yellow, brown, or cream colored with conchoidal fracture and milky luster:
If the rock is dominantly black or steely gray, relatively lightweight, with or without shiny bands, and a powder produced by scratching the material is brown to black in color:
If the rock is a siliclastic sedimentary rock and
- Has more than 80 percent sand-size or larger particles:
- Is a fine-grained sedimentary rock or has less than 80 percent sand content: