Carbonate Mineralogy Flashcards
Calcite Formula
CaCO3
Calcite Origin
Most commonly biogenic, rarely precipitated
Calcite Precipitation
Huge fans (also of aragonite and other carbonates) have formed 2 times in history:
- Precambrian
- Permian-Triassic boundary
Magnesium Substitution in Calcite
- Mg can substitute for Ca in the crystal lattice
- Leads to 2 forms of calcite found in nature
- High-magnesium (high-Mg) calcite
- Low-magnesium (low-Mg) calcite
Low-Magnesium Calcite
- contains less than 4% Mg
- most stable form of calcite
- makes stunning slides (w/o cover slip)
- autholummesence
High-Magnesium Calcite
- contains more than 4% Mg; typically between 11% - 19% Mg
- when over 19% it shifts and becomes dolomite
- High-Mg calcite is usually replaced with low-Mg calcite
Aragonite Formula
CaCO3
Aragonite Stability
Least stable, oldest known is Triassic in age; everything else has been replaced (usually by calcite) over time
Aragonite vs. Calcite
No chemical difference, but the structure is different as well as different specific gravity (aragonite 2.95 and calcite 2.72-2.94) and aragonite is more compact
Aragonite Substitution
Usually substitutes with strontium, rather than Mg, which allows for dating
Dolomite Formula
CaMg(CO3)2
-calcium-magnesium carbonate
Dolomite vs. Dolostone
-Rocks made up of dolomite are also called dolomite; some geologists will call it dolostone; usually you will need to know the context of the name dolomite
Dolomite in Hand Samples
- distinguished through the use of dilute HCl
- has little reaction on sample itself, but strong reaction when used on its streak
Dolomite Origin
Widespread but considered to be diagenetic
Siderite Formula
FeCO3
-Iron carbonate
