SULFIDES CLASS:
The Sulfide Class minerals comprise an economically important class of minerals.
Most major ores of important metals such as copper, lead and silver are sulfides. Strong generalities exist in this class.
The majority of sulfides are metallic, opaque, generally sectile, soft to average in hardness and they have high densities,
black or dark-colored streaks, and an igneous origin. But, there are a few vitreous and
transparent members such as realgar, cinnabar and orpiment that tend to break the mold, so to speak.
Minerals belonging to the selenide, telluride, antimonide and arsenide subclasses have very similar
properties to the more common sulfides and are thus included here. The whole or partial supplanting of sulfur by either selenium,
tellurium, antimony, arsenic or bismuth is possible because these elements have similar sizes, charges and ionic strengths.
Only minerals in the sulfide class that have no appreciable sulfur are included in these subclasses. If there is enough sulfur in the
mineral to be named in the formula then it is treated as a normal sulfide.
There is an exception in the case of the Sulfosalts. This is a large segment of the sulfide class whose
difference from the other sulfides lies in the position of the semi-metal ions. In most ordinary sulfides that contain a semi-metal
such as antimony, arsenic or bismuth, they substitute in the sulfur positions, but in sulfosalts they substitute for the metal ions
and bond with the sulfurs. The term sulfosalts came from a theory that these minerals were the salts of acids in which the oxygens are
replaced by sulfurs. Such as Na2SO4 is the salt of H2SO4 or sulfuric acid; then enargite, Cu3AsS4. would be the salt of the hypothetical
acid H6AsS4. This theory is not considered credible now but the name "sulfosalt" still persists.
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