Laboratory Manual for Introductory Geology 4e

6.3.2 Chemical Sedimentary Rocks
Chemical sedimentary rocks form when minerals precipitate directly from a water
solution that has become oversaturated. Ions that can no longer be dissolved bond
together to form solid mineral grains, which often display a crystalline texture as
they grow and interlock with one another. These crystals either settle out of the
solution or grow outward from the walls of the container holding the solution. In
Exercise 6.2 you can simulate the chemical precipitation process in the lab to see
how the textures of these rocks develop.
EXERCISE 6.2
Simulating Chemical Sedimentary Textures
Name:
Course:
Section:
Date:
(a) Place a beaker with seawater (or homemade saltwater) on a hot plate and heat it gently until the water evaporates.
Partially fill a second beaker with a clear, concentrated solution of calcium hydroxide [Ca(OH) 2
]. Using a straw,
blow gently into the solution until you notice a change.
Describe what happened in each demonstration and sketch the resulting textures.
(i) Evaporated seawater description
Sketch
(ii) Ca(OH) 2
solution description
Sketch
(b) Compare the texture in (i) with that of a granite, a rock formed by cooling of a melt. Describe and explain the
similarities in texture.
Groundwater, oceans, and saline lakes all contain significant quantities of dissolved
ions and are therefore sources of chemical sedimentary rocks. Precipitation
to form chemical sedimentary rocks happens in many environments, including
(1) hot springs, where warm groundwater seeps out at the Earth’s surface and cools;
(2) cave walls, where groundwater seeps out, evaporates, and releases CO 2
; (3) the
floors of saline lakes or restricted seas, where saltwater evaporates; (4) within sedimentary
rocks, when reactions with groundwater replace the original minerals with
new ones; and (5) on the deep-sea floor, where the shells of plankton dissolve to
form a gel-like layer that then crystallizes.
150 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY