Native Elements and Sulfides
This is the first lecture on systematic mineralogy beginning in Chapter 10 of your text. Before going very far you should review the layout of information listed for each mineral described in the book. This includes crystallography, physical properties, composition and structure, diagnostic features, occurrence, use and similar species.
Historically there are good reasons for classifying minerals on the basis of composition focusing first on the anion or anionic group. These include significant family resemblance in occurrence and physical features for members of the same family (all the carbonates for example), and a tendency to occur in the same or similar geologic environment.
However X-ray studies have shown that both composition and internal structure are needed to really understand and classify minerals.
We will begin with the native elements and sulfides because these are relatively
easy to understand and they show many of the features that we have been talking
about during the crystallography portion of the course.
Link to a Periodic
Table.
Key Points: Native Elements
- About 20 native elements are known exclusive of gases
- These occur as metals, semimetals and nonmetals
- The metals display cubic or hexagonal symmetries
- Ionic radii is the key to understanding solid solution in the metals
- Polymorphism is important in understanding the nonmetals: introduce the various types of polymorphic transformations (displacive, reconstructive, order/disorder)
Metals:
The metals are usually discussed in 3 groups: the gold, platinum and iron groups.
Gold Group:
- Au-Ag-Cu all belong to the same group in the periodic
table and thus have similar chemical properties.
- Isostructural with FCC lattice (Fm3m) and 12-fold coordination.
- Gold and silver have the same ionic radii (1.44 A) and thus complete solid
solution can be expected.
- Copper has radii of 1.28 A and thus has only limited solid solution.
- Metallic bonds lead to soft, malleable, ductile, sectile minerals with metallic luster and which conduct heat and electricity.
- High density results from cubic close packed structure. (How much of space
is filled by the atoms?)
Platinum Group:
- Platinum and iridium are also FCC (Fm3m) and are isostructural.
- There are also some hexagonal members of this group.
- Different geologic (and geochemical) occurrence keeps the platinum group
from encountering the gold group in the wild.
Iron Group:
- Composed of Fe, and 2 Ni-Fe alloys found in meteorites.
- Fe (1.26 A) and Ni (1.25 A) have nearly identical ionic radii and thus extensive solid solution can exist.
- Fe and kamecite (<5.5% Ni) are body centered cubic (Im3m).
- Taenite (27-65% Ni) is FCC (Fm3m).
- Kamecite and Taenite are likely to constitute a large part of the Earth's core.
Nonmetals:
Sulfur:
- Normally orthorhombic, 2 monoclinic polymorphs common in lab.
- Orthorhombic structure changes to monoclinic at 95.5 C; melting occurs at 119 C.
- Structure involves strong 8-member rings loosely bonded to other rings.
- This (i.e. weak bonds) accounts for its low melting point.
Diamond:
- Very strong tetrahedral bonding of one carbon to 4 neighbors.
- Not close packed (only 34% of space filled).
- Fairly wide spacing of carbon sheets parallel to {111} accounts for the octahedral cleavage.
Graphite:
- Strong 6-membered rings where each carbon has 3 near neighbors.
- The 4th valence electron is free to wander (metal-like) which accounts for
the excellent electrical conductivity.
- The long distance (again, weak bonds) between sheets accounts for the softness
of graphite.
- Far from closest packed - only 21% of space filled.
Key Points: Sulfides
- Majority of metal ore minerals are sulfides
- Hard to generalize about structure although most sulfides have structures
related to or derived from a few basic ones.
- Polymorphism is fairly common.
- Reasonable to consider sulfides as nearly close packed structures.
- Largely covalently bonded because generally transition metals (Periodic
Table again).
- Generally soft, pliable minerals.
- Little if any solid solution with "ionic" group Ia and IIa elements.
- Commonly difficult to 'charge balance' sulfide formulas especially when they involve semimetals such as Se and As.
Types of solid solution and structural derivations in sulfides: Substitution, omission, addition and distortion.
Look at the sphalerite structure (10-15a), the derivative chalcopyrite structure (10-15b), the derivative tetrahedrite structure (10-15c), and the wurtzite structure. What is the symmetry of chalcopyrite? Compare the sphalerite structure to that of diamond.
Look at the pyrite structure (10-17a) and compare to halite. What would you predict the symmetry of pyrite to be? Also consider marcasite (10-17b).