Lab 3: Minerals II - Halides

 

The halide minerals have one of the halogen elements in combination with another element. The halogens such as fluorine, chlorine, and bromine, have seven valence electrons, so that a good match for them is an element with only one or two valence electrons. Sodium, with its single valence electron is a direct match for chlorine, with seven valence electrons, because together, with an ionic bond, the octet rule is satisfied. Calcium, with its two valence electrons, bonds with two fluorine atoms, and likewise, the octet rule is satisfied. Here are the elements in the halide minerals presented here.

 

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Elements in halides

 

Halite (Rock Salt)

 

Chemical formula: NaCl (sodium chloride)

 

Structure: Halite has a cubic crystalline structure. Sometimes halite crystals will have an odd, stair-stepped appearance on the sides, as a result of faster crystal growth on the edges, giving rise to "salt hoppers."

 

Hammer behavior: Halite cleaves into cubes, as a result of good cleavage along three planes of weakness at 90 degrees.

 

Color: Often colorless, but also white, yellow, reddish, bluish, brown

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: transparent to translucent

 

Hardness: 2.5

 

Occurrence: Halite forms as an evaporite salt in sedimentary environments such as lake beds and marine sea bottoms, in situations where the water, whether fresh or not to begin with, becomes highly salty through evaporation. Thick salt deposits can form as a result of continued replenishment of water as it evaporates.

 

Use: Obviously, salt is useful for seasoning food, but has been important in food preservation, salting roads to melt ice in wintertime, etc.

 

Special Property: Salty taste (but be careful who you lick after)

 

Fluorite

 

Chemical formula: CaF 2 (calcium fluoride)

 

Structure: Fluorite has a cubic crystalline structure and forms cubic crystals.

 

Hammer behavior: There are four planes of weakness within the crystalline structure of fluorite, leading to very interesting cleavage. You will often see four-sided "pyramidal" cleavage faces, and in some cleaved specimens there are opposing four-sided "pyramids" forming an octahedron (eight sides total).

 

Color: Colorless, often with a purple tint, or even heavy purple, blue, yellow, green

 

Streak: white

 

Luster: vitreous

 

Diaphaneity: translucent

 

Hardness: 4

 

Occurrence: Fluorite primarily occurs in veins associated with deposits rich in metals, but can be found in igneous and sedimentary rocks also.

 

Use: Fluorite is used in glass making and ceramics, as a source for hydrofluoric acid, and is used in the manufacture of steel.

 

Special Property: Fluorescence under a black light (ultraviolet), often showing blue colors (hence, the name).