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

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)

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

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

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

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

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

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

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

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

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

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