Chemistry Reference
In-Depth Information
environment subject to high evaporation (upper intertidal and supratidal
environment) anhydrite may crystallize. It has a habit of fine-grained nod-
ules, coalescing and separated by thin walls of sediment.
Halite precipitates when no more than 10% of the initial volume of
seawater remains. It may be associated with magnesium sulfate (kieserite,
MgSO
4
H
2
O) and chloride.
Potassium (and magnesium) chlorides and sulfates are formed when
the residual water represents only 6% of the initial volume: sylvite KCl
carnalite KMgCl
3
· 6H
2
O, kainite KMgClSO
4
· 3H
2
O, polyhalite K
2
MgCa
2
(SO
4
)
4
· 2H
2
O. There are also complex diagenetic reactions between
these minerals.
The above sequence supposes a single cycle of evaporation. Evaporite
deposits are generally far from being as simple in their organization, both in
plane and vertically, due to the complex evolution of the basins and to new
seawater inputs.
Evaporites formed in a continental environment contain halite, gyp-
sum and anhydrite and specific minerals as sulfates and sodium carbonates.
Unlike marine evaporites, potassium sulfates and chlorides are rare.
In addition to the previous minerals, continental evaporites may contain
small amounts of bromides, fluorides, iodides, borates (California, Nevada,
Turkey), nitrates (Chile), lithium minerals, etc. Continental evaporite con-
tain more diverse minerals than marine evaporites because they are formed
from waters that may have leached the most varied terrains - particularly
rocks of volcanic origin - and/or may come from thermal springs.
Gypsum (and anhydrite) can also form in deep (at least below the limit
of influence of waves), partially isolated basins. That produces euxinic con-
ditions in the deepest parts of the basin. Gypsum and/or anhydrite formed in
these conditions show a laminated facies. They are associated with carbon-
ates and organic material.
When the burial exceeds several hundred meters, gypsum is completely
transformed into anhydrite. When this anhydrite reaches the surface, by ero-
sion or tectonics, it is rehydrated and transformed into gypsum. The habit
of this secondary gypsum is characteristic; large euhedral crystals, fibrous
gypsum, alabaster or massive bodies formed by small, poorly defined,
intermeshed crystals.
Gypsum is a secondary mineral developed from sulphides, particularly
in ore deposits. It can also be formed by reaction between the sulfuric acid
formed by oxidation of sulfides with carbonate rocks, limestone or marl. It
is formed in volcanic environment by the action of sulphur-bearing fuma-
roles on calcic minerals.
Anhydrite is a primary magmatic mineral in volcanic trachyandesitic
pumices (for example in Mount Pinatubo, Philippines). As anhydrite is solu-
ble, it is not preserved in ancient volcanic rocks.
Halite is also a deposit from fumaroles.
