Carbonates

Question 1

Major carbonate minerals

Answer 1

Calcite:
•	CaCO3
•	Trigonal - Rhombohedral
•	Most common carbonate mineral found in the rock record
•	Layered
•	Central Ca ion
•	2 Cleavage planes
•	Not most common carbonate mineral being made today

Aragonite:
• CaCO3
• Orthorhombic
• Most common carbonate mineral being made today
• Metastable – alters to calcite over time
o Why calcite is most abundant
• Less stable but more resistant (mechanically stronger) – why found in shells (corals and molluscs as its preferred through natural selection

Dolomite:
•	(Ca.Mg(CO3)2)
•	Not calcium carbonate – calcium magnesium carbonate
•	Not depositional – forms through alteration of calcite
•	Calcium is replaced by magnesium
o	Ratio
	Below 50% = high magnesium calcium
	Above 50% = dolomite
•	Meteoric waters the most likely supply of magnesium.
•	Increased resistance to weathering.
•	Iron causes buff colour in instances
•	Wont dissolve under acid

Breakdown:
• Aragonite
o Shimmery
o Allochems

• Dolomite
o Buff + rhomboidal
o Authochems

Question 2

Carbonate trends and sequence stratigraphy?

Answer 2

• Rapid sediment production fills TST.
• Water is going up but hasn’t reached top stage
• Making accommodation space at maximum speed
• Carbonates will follow sea level growth and lead to thick carbonate packages
• High stand + lots of accommodation space
• Limestones are biogenic so the high sea level can drown the carbonate
• Falling stage systems tract
• Sea level goes down, leaving carbonate exposed
• Fine detail of carbonate is removed, and top layer is cemented by enrichment of calcite fluid
• Now left with a protruding resistant layer – limestone pavements