EPSC 233. Compositional variation in minerals. Recommended reading: PERKINS, p. 286, 41 (Box 2-4).

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1 EPSC 233 Compositional variation in minerals Recommended reading: PERKINS, p. 286, 41 (Box 2-4). Some minerals are nearly pure elements. These are grouped under the category of native elements. This includes natural alloys, especially among metallic elements that have similar electronic structures and are not too different in their radii. Alloys tend to include metallic elements with similar electronic structures. Gold (Au) and silver (Ag) occur commonly as alloys. A natural Au-Ag alloy is called electrum. Cu, Ag, Au belong to same family. Ag, Au are most similar in size and form alloys.

2 Other native elements are not metallic but display covalent bonding. These are elements with relatively high electronegativity values. A structure dominated by covalent bonding is less tolerant of compositional variation. Atoms must be replaced by others that are not too different in size and in the type of orbitals containing the valence electrons. native carbon: natural diamond (above) & graphite (below) are remarkably pure. This is because the bonding involves sharing of electrons, and occurs along specific directions controlled by the shape and size of orbitals. Most minerals are chemical compounds. Their composition is described by definite proportions of two or more chemical elements. In some cases, the composition of minerals varies very little from an ideal formula. Analyses of minerals such as quartz (SiO 2 ), aluminosilicate polymorphs (Al 2 SiO 5 ), corundum (Al 2 O 3 ), show deviations of less than 0.5% from their exact formula. Other minerals are far more variable The overall proportions of certain types of elements are respected, but some elements may be nearly interchangeable. This is noted, in the chemical formulas of some minerals, by grouping these interchangeable elements within parentheses. (Mg, Fe) 2 SiO 4 olivine

3 The name olivine refers to a group which includes many members of intermediate composition. Minerals like forsterite (Mg 2 SiO 4 ) are part of solid solutions. Ternary diagram: includes 3 end-member compositions. The chemical variations found within specific solid solutions can be displayed graphically. The example below is a binary solid solution. Possible compositions include intermediate values between 2 end-members (pure compounds) forsterite and fayalite: The main method of analyses of single minerals in electron probe microanalysis (EPMA). The chemical analyses obtained by the electron microprobe are given in weight % oxides. It is necessary to recalculate the amounts to figure out how they fit the proportions expected from the formula unit. Your next assignment will be a recalculation of a chemical analysis into a mineral formula. Pure diopside In this example, the corners of the triangles are the formula of pure minerals. Shading indicates the range of compositional variation.

4 In most cases, a 50:50 rule applies to naming members of a solid solution. For example, the name forsterite is used for compositions going from Mg 2 SiO 4 up to MgFeSiO 4. The name fayalite is used for compositions that are more Fe-rich than MgFeSiO 4, and up to Fe 2 SiO 4. Feldspars have names for much narrower ranges of composition. Most cannot be identified in hand specimens, because the composition does not affect noticeably their physical properties. Why are some minerals more variable in composition than others? During crystal growth, the environment contains some chemical elements in forms than are fairly interchangeable. Some ions are so similar in size and charge that they can fit in the same atomic pattern. Mg 2+ and Fe 2+ are the most common example. Their ionic radii, when they are surrounded by 6 oxygen ions (C.N. VI) are 0.72 and 0.78 Å. Substitution can lead to complete solid solution. For small differences in ionic radii 100% x (Radius large ion -Radius smaller ion )/Radius large ion In principle, if this difference < 15% complete solid solution is common = 15-30% limited solid solution is possible > 30% substitution is very limited Other factors may play...

5 What properties of the mineral structure are affected by ionic substitutions? Hardness? Specific Gravity? Colour? Melting Point? Ease of weathering? (solubility in acidic water) (Think of forsterite-fayalite) What properties of the mineral structure are affected by ionic substitutions? Hardness? Sometimes, if bond strength changes. Specific Gravity? Often, if elements have different atomic masses. Colour? Often. Even small amounts of transition elements may influence colour. Melting Point? Sometimes, if bond strength changes. Ease of weathering (solubility in acidic water)? Sometimes, this tends to increase with the ionic character and bond length. Effect of compositional variation on colour in garnet Degree of ionic character of the bond changes from Mg-O to Fe-O Look at the electronegativity values of Mg, Fe: Mg: 1.31 Fe : 1.83 O : 3.44 radius vi Mg 2+ :0.72 angstroms radius vi Fe 2+ : 0.78 angstroms A= Ca 2+, Mg 2+, B= Al 3+, Si 4+ Fe 2+, Fe 3+, Mn 3+, Cr 3+,... Garnet structure A 3 viii B 2 vi (Si iv O 4 ) 3 Does Mg-O or Fe-O have more ionic character? Mg-O, and it will also be more soluble in water. Is Mg-O or Fe-O the shorter bond? Mg-O, and it will have the higher melting point.

6 The fact that complete solid solution is possible can be verified in the laboratory. Nature does not always produces all the possible range of composition. One can cook up in the lab forsterite, fayalite and their intermediates: Melt MgO + SiO 2 + FeO and let it cool slowly... Yet, in nature, all intermediate compositions are rare Most olivine is close to forsterite because Fe is taken up by other minerals which precipitate from the silicate magma as it cools down. An example of very limited solid solution: Hematite and corundum share exactly the same type of structure. In the first case, Fe 3+ is the cation, in the other, Al 3+. Their coordination number is 6 in each structure. According to your table of ionic radii: Fe 3+ : 0.64 (VI) Al 3+ : 0.51 (VI) Is any solid solution expected? To answer, see if the radii differ by more than 15% or 30%. Corundum (Al 2 O 3 ) and hematite (Fe 2 O 3 ) share the same structure. Note the edge sharing and face sharing among MeO 6 octahedra. This limits the flexibility of the structure towards substitution among ions of different radii. Polyhedra that share edges are commonly less tolerant of substitution than polyhedra that share corners. Nature of bonding is also different for Al-O and Fe-O Unpaired electrons are present in dorbitals, in ions of Fe (and many other transition metals), but absent in Al 3+. Despite a larger radius difference, Al 3+ and Si 4+ substitute for each other in the tetrahedral polyhedra of several tectosilicates (e.g., the feldspars). These tetrahedra share only corners.

7 When a substitution takes place, in a mineral structure, electrical neutrality must be maintained. In feldspars, this is illustrated by members of the plagioclase series: Na Al Si 3 O 8 CaAl 2 Si 2 O 8 Na + is replaced by Ca 2+ Si 4+ is replaced by Al 3+ When three or more ions are involved in the substitution to preserve electrical neutrality, we deal with a coupled substitution. Any substitution can be expressed as an equation: In the case of Na Al Si 3 O 8 -CaAl 2 Si 2 O 8 Na + +Si 4+ = Ca 2+ + Al 3+ Na + +Si 4+ = Ca 2+ + Al 3+ In some high-temperature polymorphs of SiO 2, small amounts of Al3+ and Na+ can be found. Si 4+ = Na + + Al 3+ Which ion, Na + or Al 3+, is substituting for Si 4+? (Hint: Which one is closest in size and charge? Where does the extra ion go, in the quartz structure? Interstitial substitution: There are many polymorphs of SiO 2, stable at different ranges of temperature and pressure. Those stable at highest temperature have the lowest density. Their SiO 4 groups form networks with the most open space. They are more likely to accept impurities (small amounts of Al 3+ and Na + for Si +4 ). - an extra ion must be added in the structure to respect charge balance: A x = B y + C z This type of substitution occurs in very small amounts in any structure, and is limited to structure with channels or cages.

8 Be 3 Al 2 Si 6 O 18 (beryl) has space within channels formed by the Si 6 O 18 rings. The charge balance can be met by a coupled substitution which introduces new ions within the channels. Aquamarine (blue) : Fe 2+ Heliodor (golden) : Fe 3+ Green beryl : mixtures of Fe 2+ and Fe 3+ Morganite : Mn 2+ is pink Red beryl : Mn 3+ is red Emerald: Cr 3+ is emerald green! Where do these ions go in the structure of Be 3 iv Al 2 vi Si 6 O 18? Why are they often accompanied by Li +, Na + or K +? Where does the other ion go? Some minerals have more space than other, in their structure, to accommodate the extra ions involved in a coupled substitution. The high-temperature polymorphs of quartz, minerals like cristobalite and tridymite, are less pure than quartz formed at lower temperature. Cyclosilicates, like beryl, contain large channels and can accept large impurities. Omission substitution: charge balance requires that a site normally occupied by an ion becomes empty (vacant). A x + B y = C z + [empty site] The structure and formula of pyrrhotite) are related to those of troilite, FeS, by omission substitution: Troilite Fe 2+ S 2- (found mostly in meteorites) On Earth, even low oxygen levels oxidize some Fe 2+ to Fe 3+. As pyrrhotite forms, this happens: 3 Fe 2+ = 2 Fe 3+ + [empty site]

9 Overall, pyrrhotite is Fe 1-x S because (Fe 2+ ) 1-3x (Fe 3+ ) 2x [ ] x S 2- Charge balance is satisfied: positive charges 2*(1-3x)+3*(2x) = 2 negative charges Note: the value of x (proportion of empty sites per formula unit) in natural pyrrhotite is quite small. You cannot have extensive omission solid solution and preserve a stable structure. Are we more likely to see substitution among ions located in the same family, within the periodic table? Why (or why not)? Ion Radius C.N. = 4 Radius C.N. = 8 Li Na K Rb Cs Yes, but they will occur most often between the elements that show the smallest relative differences in ionic radii. In feldspars, there is complete solid solution between NaAlSi 3 O 8 (albite) and KAlSi 3 O 8. In micas, this solid solution is far more limited. There is no Na- equivalent to biotite K(Mg, Fe) AlSi 3 O 20 (OH) 2 but a small amount (1%) of Na is common in the structure. Rb is a much rarer element, and it has a radioactive isotope which decays to Sr. Small amounts of Rb substitute for K in mica and feldspar, where it is analyzed to date rocks. Are we likely to see substitution among elements located in the same row of the periodic table? Why (or why not)? Ion Radius Radius C.N. 4 C.N. 6 Na M g Al Si P S S Cl

10 Substitutions among elements in the same row do not lead to complete solid solution as often as do substitutions among elements in the same family. This is because they are different both in charge and in ionic radius. The structure must solve the charge balance problem by another (coupled) substitution. Plagioclase feldspars are an example where Al 3+ and Si 4+ may substitute for each other if Ca 2+ and Na + ions do the same in the structure.