Evaporites Flashcards
Sed Rec
What are Evaporites?
chemical sediments precipitated out of solutions by
evaporative concentration of dissolved salts
How many evaporite minerals are there?
More than 100
What are evaporates mostly made of?
Highly soluble ions (such as Na, Cl)
What are the main evaporite minerals?
gypsum, anhydrite, halite
What are the subordinate evaporite minerals?**
Mg & K salts, natron, and borax
TRUE OR FALSE
Some evaporite rocks are mono-mineral
TRUE! Examples are: gypsum, halite, etc.
Describe Halite
(chemical makeup, color, size, etc)
NaCl - cubic crystals (sometimes hooper-type),
granular, or massive colorless or white, sometimes
exotically colored
In evaporites, precipitation of halite leads to the developing
of:
- Marine and non-marine evaporite deposits
- Different colors depending on various lattice disorder or
- presence of impurities (e.g., K+ gives a blue color)
- Isotropic in thin section (as it is cubic), shows good
- cleavage, and often fluid inclusions;
- Hooper crystals (crater like): appear when a crystal grows
- faster at the edges of each face than at the center
- Happens when the growth is fast or/and not enough material to fill in the inner part
Describe Gypsum
(chemical makeup, color, size, etc)
CaSO 4 · 2H 2O - monoclinic; prismatic crystals or
aggregates when accumulates in massive beds
Describe Anhydrite
(chemical makeup, color, size, etc)
CaSO4 - orthorhombic, fibrous, massive, or nodular.
Colorless to pale blue or violet if transparent, white or brown if impurities are
present within crystal lattice Marine and non-marine evaporite
Where do Anhydrites form?
in restricted sub-aqueous and deep burial environments by dehydration
of gypsum at >22ºC mean annual temperature
TRUE OR FALSE
The texture of gypsum-anhydrite vary considerably
TRUE! Textures vary depending on their precipitation environment and diagenetic history
What does Gypsum look like under a cross polarized microscope?
weak birefringence and low relief
What does Anhydrite look like under a cross polarized microscope?
higher relief and moderate birefringence
Describe Trona
(chemical makeup, color, size, etc)
Na3(HCO 3)(CO 3)· 2H 2O - monoclinic, forming well-developed columnar and fibrous crystals or massive aggregates
Is Trona marine or non-marine and when is it deposited?
- Non-marine evaporite
- Deposited in the latest stage of a salt lake evolution
Trona is mined as the primary source of _?
Sodium Carbonate
What are some uses of Trona?
- manufacturing of glass, paper, detergents, and food additive
- fertilizer (potash)
Describe Borax
(chemical makeup, color, size, etc)
hydrous sodium borate Na2B 4 O 5 (OH) 4 · 8H 2O - monoclinic, forming well-developed prismatic crystals or earthy aggregates
Is Borax marine or nonmarine and what are some uses of it?
- Non-marine
- Used for detergent, cosmetics, enamel
Depositional Environments
Evaporite deposits are commonly
cyclic
How many modern sites of evaporite precipitation are there?
Few and none of them scale to what existed in the past
What are the 5 different environments (see figure at right) in which
deposition of evaporites take place in?
- Marine: deep (shabka/lake/lagoon; 1-2-3)
- Shallow water (intracratonic/rift basin; 4)
- Non-marine: takes place within sediment (sabkha;2) or in very shallow to desiccated saline pans (5)
Has taken place on the floor of deep-marine basins following evaporative drawdown and extreme desiccation
EX: Mediterranean basin
What does Sabkha mean?
Salt flat (arabic)
