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How to Identify Minerals in 10 Steps

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7 of 10

Step 7: Cleavage and Fracture
Watch the pieces

How minerals break is a key clue to their identification.

Photo (c) 2011 Andrew Alden, licensed to About.com (fair use policy)
Cleavage is the way a mineral breaks. Many minerals break along flat planes, or cleavages—some in only one direction (like mica), others in two directions (like feldspar), and some in three directions (like calcite) or more (like fluorite). Some minerals, like quartz, have no cleavage. Cleavage is a profound property that results from a mineral's molecular structure, and cleavage is present even when the mineral doesn't form good crystals. Cleavage can also be described as perfect, good or poor.

Fracture is breakage that is not flat. The two main kinds of fracture are conchoidal (shell-shaped, as in quartz) and uneven. Metallic minerals may have a hackly (jagged) fracture. A mineral may have good cleavage in one or two directions but fracture in another direction.

To determine cleavage and fracture, you'll need a rock hammer and a safe place to use it on minerals. A magnifier is also handy, but not required. Carefully break the mineral and observe the shapes and angles of the pieces. It may break in sheets (one cleavage), splinters or prisms (two cleavages), cubes or rhombs (three cleavages) or something else.

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