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What is the structure of fluorite?

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Fluorite is composed of calcium and fluorine atoms arranged in a simple cubic, crystalline lattice structure. Calcium atoms in fluorite are arranged in a face-centered cubic arrangement embodying a cube of fluorine atoms. In other words, fluorite is composed of a cubic array of fluorine atoms with a calcium atom in the center of every alternate cube.

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A simple cubic structure such as that of fluorite has equal spots for atoms and holes. Fluorite possesses a cubic arrangement of fluoride ions occupying the spots for atoms with calcium ions inserted in the cubic holes. The ionic radius of calcium ion is only slightly higher than the ionic radius of fluoride. The similar ionic radii makes it unimportant whether fluorite structure is discussed in terms of fluoride ions occupying holes in a calcium ion lattice or vice versa. The ion with smaller radius is expected to prefer cubic holes.

Imbalance between charges on calcium (-2) and fluoride ions (-1) prevents calcium ions from occupying every hole in the cube without altering the overall charge on fluorite. Subsequently, to maintain the electroneutrality of fluorite, only half of the cubic holes in a fluoride array are occupied by calcium ions. This results in a ratio of 2:1 for fluoride ions to calcium ions in the resultant fluorite structure. The arrangement of ions in this pattern results in diagonal planes of weakness within cubes where cations are absent. These planes of weakness account for four directions of perfect cleavage in fluorite mineral when broken. It is the only mineral which frequently breaks into octahedral-shaped pieces owing to its structure.

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