Download Sedimentary Rock Identification Dichotomous Key

Sedimentary Rock
Identification Dichotomous Key
A. DICHOTOMOUS KEY TO THE SEDIMENTARY ROCKS
Generally speaking, sedimentary rocks are either made of other rocks NOT minerals, sand-sized or dull and
earthy. The primary classification is chemical or clastic followed by the texture (grain size) & composition
(minerals). For example, if a rock creates bubbles with vinegar is dropped on it, then it most likely contains the
mineral that effervesces with vinegar such as calcite. If a sandstone is dominated by sand-sized pieces of
quartz, then most likely it will scratch glass. Use the descriptions of the three categories of sedimentary rocks
(Chemical, Organic and Clastic) to guide your through the identification process.
1a. Made of rock or mineral particles of sand size or larger – (2)
1b. Not composed of rock or mineral particles, or, if present, smaller than sand size – (7)
2a. Particles of pebble size or larger – (3)
2b. Particles of sand size – (4)
3a. Mostly rounded particles in a matrix of smaller materials Conglomerate
3b. Particles mostly angular(sharp) –Breccia
4a. Coarse; angular particles of pink feldspar; generally pinkish to brown – Arkose
4b. Feldspar usually not present, color varies – (5)
5a. Particles spherical, smooth, pinkish/grayish; effervesces in vinegar –Limestone
5b. Particles not spherical – (6)
6a. Dark gray to greenish; finer grained, with some clay particles – Graywacke (do not have)
6b. Color varies, but usually light; rounded to subrounded(rough) grains – Sandstone
7a. Composed of intergrown crystals – (12)
7b. Not composed of visible crystals – (8)
8a. Waxy luster, harder than glass – (9)
8b Luster otherwise; softer than glass (4.5 on Mohs scale) – (10)
9a. Dark gray to black – Flint (don’t have)
9b. White to medium gray – Chert
10a. Effervesces in dilute acid (11)
10b. Does not effervesce in acid (14)
11a. Dense, few crystals visible, conchoidal fracture – Micrite Limestone (do not have)
11b. Otherwise (13)
12a. Effervesces in dilute acid - Crystalline limestone or Organic Limestone
12b. Does not effervesce in acid and clear or multi-colored and within a larger dull rock- Geode
13a. Composed of visible shell fragments – Coquina (yellowish in color; “rice krispy”)
13b. Soft, can be scratched with fingernail – Chalk or Kaolinite (don’t have)
14a. Dense, shiny black, low specific gravity (therefore lightweight) – Bituminous Coal
14b. Otherwise (15)
15a. Fine-grained; fissile (layered look); color variable and usually has fossils – Shale
15b. Massive (not fissile) earthy luster – Mudstone (Siltstone/Claystone)