Ask-a-Geologist #14: What are the Properties of Calcite?

What are the properties of calcite?

Calcite is very popular and we get lots of great questions about this unique mineral. Sophia wrote in and asked about the properties of calcite. This is a fun one!

What are the Mineral Properties of Calcite?

Sophia asked, “What are the properties of calcite? I’m especially interested in its cleavage directions and fracture pattern.”

Calcite is a common mineral but has some unique properties. You can find calcite in most geologic settings around the world. Calcite is the main component of the sedimentary rock limestone and the metamorphic rock marble. The mineral will dissolve in an acid which is why many caves form in massive limestone terrains when the mineral dissolves due to acid rain. Here are a few calcite fun facts:

  • Chemical Formula – CaCO3 – calcium carbonate
  • Many colors; white, purple, yellow, red, orange, blue, green, brown, and clear
  • Hardness: Very soft, 3 on Mohs Hardness Scale
  • Luster: Vitreous (Glassy) to Pearly
  • Streak: White
  • Specific Gravity: 2.71 g/cm3
  • Cleavage: Three (3) directions of perfect cleavage means that it will break smoothly along cleavage planes, and forms a rhombohedral shape
  • Fracture: Conchoidal
  • Reactive to acids – geologists use the “fizz test” to check for calcite

Varieties of Calcite

White rhombic calcite sample

Not all samples of calcite will have that perfect rhombus shape. In the video, I show you three samples of calcite. One of the samples is not a perfect rhombus but gives you hints of the cleavage.

Iceland spar calcite sample

Iceland spar calcite is a special clear variety. Its rhombohedral shape and clarity give it a unique optical property. When you put a sample of the Iceland Spar Calcite over text or an image it appears doubled through the crystal.

I hope this gives you some fun information and a new appreciation for the mineral calcite.

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