Notes for Use of the Basic Key
For Sedimentary Rock Identification
If you are unfamiliar with the use of keys for sedimentary rock identification, or unfamiliar with some of the properties tested for, the following notes may help you get started, and avoid the common pitfalls.For Sedimentary Rock Identification
1. Keys and Classifications are not the same thing. A classification groups together objects by some theoretical principle; see, for example, discussion of basic classification. A key is just a method of identification, and subdivides objects only for that purpose. Do not expect the classification and key to match up well.
In this key, clastic, chemical and biochemical rocks are often intermixed. The rock names are color coded to their origin, however, according to the following scheme.
2. Rocks are identified using a key by making a series of observations, and decisions. If these are made correctly then the path on the key will lead you to an identification. If a wrong observation or decision is made then the path will lead you to the wrong identification. For example, the portion of the key below requires a number of observations: grain size, conchoidal fracture, hard enough to scratch fingernail, etc.
The degree to which you know these things, or are able to make these observations determines the usefulness of the key. Some of it is common sense, and some of it is just experience; learning, for example, what is a scratched fingernail, and what is not. More information on some of these is below.
The key is devised, however, where ever possible, to give you several pieces of evidence to make your identification, such as acid reaction, hardness, grain size, and other properties. To check your identification make sure the specimen conforms to all the properties laid out.
3. Two of the main criteria used in sedimentary rock identification is acid reaction (which separates carbonate from chemical and biochemical specimens), and whether the rock is hard enough to scratch glass or not. Observe that the key brackets acid reaction on the left and hardness on the right, and that they do not match up with each other. Each is briefly explored below.
Acid Reactions:
To test acid reaction we use a dilute solution of hydrochloric acid (10% or less). This will still eat holes in your cloths if you get it on you, so wash the specimen well.
A carbonate rock should react vigorously to a drop of the acid (big foaming bubbles), with the exception of dolomite. Dolomite is a calcium magnesium carbonate and must be well powdered to get even a weak reaction (fine bubbles that develop slowly). Note, however, that limestone and dolomite intermix completely, so any weak acid reaction usually means dolomite.
The other caution is, many rocks are contaminated with lime, especially clastic rocks, and they may react vigorously anyway. You will have to observe carefully to separate limy clastics from carbonates.
Hardness:
Hardness is a common scale in geology, especially for minerals where the 10 step Moh's hardness scale is used, although in most cases in comes down to whether the mineral is harder or softer than glass, a penny, or a fingernail.
In this key hardness is measured as greater or softer than glass, and the fingernail.
For glass hardness we use plate glass (window glass) in about 2 inch squares, laying it down on the table and rubbing the specimen firmly across the glass. It is good to test the specimen in several places. And, make sure that the glass has been scratched and the rock not just rubbed off. Run your fingernail across the glass to see if you can feel a nick.
4. Other properties that may be unfamiliar.
Conchoidal Fracture:
This is when the specimen breaks along smooth to irregular curving surfaces, such as in quartz or obsidian. It is a property of uniform materials, and often shows up in rocks such as chert, micrite limestone, shale, and others.
Grain Size:
Grain size is a crucial property for the identification of most rocks. At first people learn it by using a ruler or a grain size scale, and after that is becomes natural.
Two of the major size boundaries are between sand and gravel, and between sand and silt. The sand-gravel cutoff is 2mm, and can usually be determined with a ruler. The pdf version of the key has 2mm diameter circles in the key for identification.
The sand-silt boundary is harder to determine because silt is less than 1/16 mm, which is pretty small to see by eye. A good rule of thumb is if you can see the individual grains distinctly, even with a microscope it is sand. If the grains get too small to clearly distinguish from each other then call it silt.
As for silt and shale, that is the fingernail test. Silt distinctly scratches a fingernail; shale just slips across the surface.
Color:
Color is one of the first properties that strikes the eye when examining a rock. But, Never Trust Color! Sedimentary rock color varies enormously, and often has little or nothing to do with how the rock is classified. Color often is, however, a good indicator of the depositional environmental conditions.
Still, don't let the color of a rock influence your observations and decisions; it will get you into trouble more times than not.
5. Contaminated specimens, or impure specimens are frequent. Sometimes a rock which is not supposed to scratch glass, like a shale, will be contaminated with a small percentage of sand grains, not enough to notice at first. These will scratch the glass, and can lead you astray. Do the tests carefully, and test more than one spot on the specimen.
And, of course, reaction with HCl. It is very common for clastic rocks to be contaminated with calcite, and distinguishing a clastic rock even though it is reacting it one of the important things to learn.
6. There is no right or wrong key. A key is judged by its usefulness at separating out rock types, especially those that are confusing or difficult to separate.
A key is a start, but it is not infallible. With experience you may be able to devise keys that is more useful to you, or ones that discriminates more kinds of rocks than this key does.
For help in using keys better, and learning how to study well to learn specimens, go to Hypothesis Testing.
Return to Basic Key Return to "A Basic Sedimentary Rock Classification" Pdf version of the Key