1. Halides, Sulfates, and Borates
GLY 4200
Lab 7 - Fall, 2017

2. Halides Minerals contain the anions F-, Cl-, Br-, or I-
More than one hundred forty halides are known – few are important
Bonding is usually nearly purely ionic
Anion is spherical, so structures have high symmetry
If the cation is small and highly polarizing, symmetry will be reduced (cryolite, Na3AlF6 is monoclinic)

3. Halide Properties Ionic bonding makes them highly soluble
Usually found in evaporite deposits in arid regions
Some are quite insoluble
Isometric halides have low hardness and moderate to high melting points
They are poor conductors of heat and electricity except near the melting point

4. Taste Halite (NaCl) and sylvite (KCl) are similar in appearance
One method of distinguishing them is taste
Halite = salty
Sylvite = bitter

5. Sulfates
There minerals are characterized by cations bonded to the SO42- group
This group is tetrahedral
The sulfur-oxygen bonds are strong, and the anionic group does not break up during most chemical reactions
Exception: Anaerobic bacteria are capable of breaking the sulfur-oxygen bond

6. Sulfate Groups There are two major groups of sulfate minerals
Hydrous Sulfates
Barite (anhydrous)

7. Barite Group Large divalent cations in XII coordination
Similar structures in manganates (MnO42-) and chromates (CrO42-) with large cations
Barite is usually found in hydrothermal veins, often as a gangue mineral with ores of Ag, Pb, Cu, Co, Mn, and Sb
Used as drilling mud because of its high density

8. Anhydrite Anhydrite has a much smaller cation, Ca2+
It has a CN of VIII
Found in locations similar to gypsum
Cap rock of salt domes
In limestones
In amygdaloidal cavities of basalts, as a secondary mineral

9. Gypsum Group Gypsum is hydrous
Has a layered structure, with layers of sulfate bonded to calcium
Sheets of water molecules separate the Ca-SO4 layers
Bonding between water molecules is weak, producing excellent cleavage

10. Varieties of Gypsum Satin spar – fibrous, silky luster
Alabaster – fine-grained, massive
Selenite – broad, usually colorless and transparent, cleavage folia
Twinning is common
Occurrence: Often in thick-bedded sedimentary deposits, may be interstratified with limestone and shale
Often under beds of rock salt (halite) in evaporite deposits

11. Borates Forms extended polymer structures
Trigonal planar, BO33-
Tetrahedral, BO45-
Extended network of BO3 triangles is possible, but has low stability
Make it useful in glass preparation for light weight, highly transparent glasses

12. Borate Occurrence
Many borates are found in evaporite deposits in arid regions
Most are hydrous, and will be destroyed if completely dehydrated
May be deposited from brine solutions

13. Text Reference
See chapter 16 in the text for more information on halides, pp and ; see chapter 17 in the text for more information on borates, pp. 403 and ; see chapter 17 in the text for more information on sulfates, pp and