Calcite

Calcite is a form of calcium carbonate. It’s chemically identical to aragonite, but its calcium and carbonate ions are arranged differently.

Calcite has a trigonal crystal structure. Aragonite’s is orthorhombic. That difference makes calcite the more brittle of the two.

Why It’s Important

• Corals should build their skeletons from strong aragonite, not brittle calcite.
• Aragonite holds up better to pH swings and is harder and denser, which is why it’s the form you want in a reef tank.
• When calcium, alkalinity and magnesium drift out of balance, corals may form brittle calcite or grow more slowly.
• Potassium, used with boron, can help corals produce strong aragonite instead of brittle calcite.

How It Works

The Aqua Care Guide™ uses diamond and graphite as the comparison. They’re exactly the same chemically, yet diamond is far stronger because of how it’s bonded.

Calcite and aragonite work the same way. Same compound, different arrangement, different strength.

Calcite and Related Terms

Calcite and aragonite are both calcium carbonate. Which one corals lay down during calcification depends on a balanced supply of calcium, alkalinity and magnesium, with potassium and boron helping too.

Common Misconception

Because calcite and aragonite are both calcium carbonate, it’s easy to treat them as the same thing. Chemically they are. The crystal structure is what makes aragonite the stronger one.

The Bottom Line

Calcite is the brittle form of calcium carbonate, and keeping CAM in balance helps corals build aragonite instead. More in The Science Behind Aragonite.