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Halides, Sulfates, and Borates

GLY 4200

Lab 7 - Fall, 2012

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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)

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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

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Taste

• Halite (NaCl) and sylvite (KCl) are similar in appearance

• One method of distinguishing them is taste Halite = salty Sylvite = bitter

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Sulfates

• There minerals are characterized by cations bonded to the SO4

2- 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

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Sulfate Groups

• There are two major groups of sulfate minerals Hydrous Sulfates Barite (anhydrous)

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Barite Group

• Large divalent cations in XII coordination• Similar structures in manganates (MnO4

2-) and chromates (CrO4

2-) 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

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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

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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

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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

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Borates

• Forms extended polymer structures Trigonal planar, BO3

3-

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

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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

Text Reference

• See chapter 16 in the text for more information on halides, pp. 374-375 and 393-397; see chapter 17 in the text for more information on borates, pp. 403 and 416-419; see chapter 17 in the text for more information on sulfates, pp. 403-405 and 420-427

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