CHEMICAL
SEDIMENTARY ROCKS
Prepared by Dr. F. Clark
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
INTRODUCTION
Chemical sedimentary rocks would be those whose grains
formed by direct precipitation or crystallization from
water, without organic influence or mediation. This latter
qualification is somewhat problematic, given that most
bodies of water have organisms which will influence the
chemistry of that water at least to some extent. There is
one volumetrically and economically significant group of
sedimentary rocks of non-controversial chemical origin,
the so-called evaporites. As their name implies, they
form by evaporation of concentrated sea or saline lake
water.
EVAPORITES
As a body of sea water or a saline lake experiences net
evaporation, the concentration of the ions dissolved in
that water rises until the saturation point of various
materials is exceeded, and minerals precipitate or
crystallize. Many of these minerals are economically
significant, such as gypsum, halite, and potash salts
from sea water, and epsom salts, borax and trona from
saline lakes. The first minerals to form as the water
evaporates are carbonates, which we have covered
already under biochemical sedimentary rocks. They are
generally volumetrically minor components of evaporite
mineral assemblages.
Gypsum.
This is
hydrated
calcium
sulphate; the
sulphates are
the second
major group to
form as sea
water
evaporates.
Such large crystals as these are frequently formed by precipitation from
saturated groundwater circulating through near-surface sediment
deposits and soils, rather than precipitation from sea water. Note the
clarity of these large crystals, which have a Mohs hardness of 2.
Gypsum – the Effect of Crystal Size
Both photos illustrate the effect on opacity that crystal size has. These
evaporite samples consist of thousands of individual small crystals,
whose edges and grain boundaries dominate the optical effects and
render the samples opaque, even though gypsum is transparent to
translucent. The right sample is from the Devonian age Elk Point
Group of the Western Canada Sedimentary Basin.
Anhydrite.
This is calcium
sulphate
without the
bound
molecular
water that
defines
gypsum.
In this sample
it is white.
This is less likely to form as a primary evaporite mineral, because the
presence of water makes gypsum formation more likely. It is possible
to dehydrate gypsum after its initial formation, or as apparently
happened in this case, for anhydrite to form in a carbonate host rock.
Halite – Rock Salt, NaCl
Halite forms third in the sequence of evaporation of sea water, and in a
closed system would account for approximately 75% of all the solids
that will form. It may be mined conventionally, or as suggested by
the core sample on the right, can be recovered from bore holes by
circulation of water in the subsurface and evaporation of the
resulting brines at surface. The salty taste is distinctive.
Sylvite.
This is
potassium
chloride (KCl),
and unlike
halite, has a
distinctly bitter
taste. Its other
properties are
similar to those
of halite.
The fourth group to form from the evaporation of sea water is a
complex assemblage, whose most useful member is sylvite. It is found
in the potash deposits of Saskatchewan, and is a major component in
fertilizer production. It forms when 98% of the water is lost.
CHERT – PROBLEMATIC ORIGIN
Chert, a microcrystalline or cryptocrystalline form of silica
(SiO2), can originate either by organic means or not.
There are two principal groups of organisms that secrete
siliceous skeletons whose accumulation can result in the
formation of bedded chert. Radiolarians are microscopic
plankton that are less than 1 mm in size. As well, there
are some sponges whose spicules (tiny hard parts that
support the tissues) are siliceous. These groups first
appeared in the Cambrian, so older rocks (>544 million
years old) hypothetically can’t be biochemical. Even
younger chert may be chemical in origin, forming during
diagenesis as nodules and beds, usually in carbonates.
Chert.
Chert has
many
properties in
common with
quartz. It has a
hardness of 7,
and samples
exhibit
conchoidal
fracture.
There are many coloured varieties of chert, ranging from white through
yellow and pale green to black, depending on trace elements and their
chemical state (e.g. red for oxidized and green for reduced iron).
Chert from Western Canada
The occurrence of chert as beds or bands, especially in carbonate
sections, is quite common in the rocks of the Western Canada
Sedimentary Basin. From left to right we see examples from the
Mississippian Rundle Group, the Pennsylvanian Kananaskis Formation,
and the Permian Ishbel Group. Not only is chert common in these
rocks, but when they are weathered after uplift during building of the
Rocky Mountains, they form a rich source of the common chert pebbles
and sand grains seen in Cretaceous age siliciclastic units.