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EPSC 233 Compositional Variation in Minerals Recommended Reading

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EPSC 233 Compositional Variation in Minerals Recommended Reading EPSC 233 Some minerals are nearly pure elements. These are grouped under the category of Compositional variation in minerals “native elements”. This includes natural “alloys”, especially Recommended reading: among metallic elements that have similar electronic structures and are not too PERKINS, p. 286, 41 (Box 2-4). different in their radii. Alloys tend to include metallic elements with similar electronic structures. Gold (Au) and silver Cu, Ag, Au belong to same family. Ag, Au (Ag) occur commonly are most similar in size and form alloys. as alloys. A natural Å Au-Ag alloy is called electrum. 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 native carbon: natural diamond (above) & graphite different in size and in the type of orbitals (below) are remarkably pure. containing the valence electrons. 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 Other minerals are far more variable… proportions of two or more chemical elements. The overall proportions of certain types of In some cases, the composition of minerals elements are respected, but some elements varies very little from an ideal formula. may be nearly interchangeable. This is noted, in the chemical formulas of Analyses of minerals such as quartz (SiO2), some minerals, by grouping these aluminosilicate polymorphs (Al2SiO5), corundum “interchangeable elements” within (Al2O3), show deviations of less than 0.5% from their exact formula. parentheses. (Mg, Fe)2SiO4 olivine The name “olivine” refers to a group which includes Ternary diagram: includes 3 end-member compositions. many members of intermediate composition. Minerals like forsterite (Mg2SiO4) are part of solid solutions. 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: In this example, the The main method of analyses of single corners of the triangles minerals in electron probe microanalysis are the formula of pure (EPMA). minerals. Shading indicates the range of The chemical analyses obtained by the Pure compositional variation. electron microprobe are given in weight % diopside 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. Feldspars have names for much In most cases, a “50:50 rule” applies to naming narrower ranges of members of a solid solution. composition. Most cannot be identified For example, the name “forsterite” is used in hand specimens, for compositions going from Mg2SiO4 up to because the MgFeSiO4. The name “fayalite” is used for composition does not compositions that are more Fe-rich than affect noticeably MgFeSiO4 , and up to Fe2SiO4. their physical properties. Why are some minerals more variable in Substitution can lead to complete solid solution. composition than others? For small differences in ionic radii During crystal growth, the environment 100% x (Radiuslarge ion -Radiussmaller ion)/Radiuslarge ion contains some chemical elements in forms than are fairly interchangeable. In principle, if this difference < 15% complete solid solution is common Some ions are so similar in size and charge = 15-30% limited solid solution is possible that they can fit in the same atomic pattern. > 30% substitution is very limited Mg2+ and Fe2+ are the most common example. Other factors may play... Their ionic radii, when they are surrounded by 6 oxygen ions (C.N. VI) are 0.72 and 0.78 Å. What properties of the mineral structure are What properties of the mineral structure affected by ionic substitutions? are affected by ionic substitutions? Hardness? Sometimes, if bond strength changes. Hardness? Specific Gravity? Often, if elements have Specific Gravity? different atomic masses. Colour? Colour? Often. Even small amounts of transition Melting Point? elements may influence colour. Ease of weathering? Melting Point? Sometimes, if bond strength (solubility in acidic water) changes. Ease of weathering (solubility in acidic water)? (Think of forsterite-fayalite) 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 radius viMg2+:0.72 angstroms Fe : 1.83 radius viFe2+: 0.78 angstroms O : 3.44 Does Mg-O or Fe-O have more ionic character? Mg-O, and it will also be more soluble in water. A= Ca2+, Mg2+, B= Al3+, Si4+ Garnet structure Is Mg-O or Fe-O the shorter bond? viii vi iv Fe2+, Fe3+, Mn3+, Cr3+ , ... A3 B2 (Si O4)3 Mg-O, and it will have the higher melting point. The fact that complete solid solution is possible An example of very limited solid solution: can be verified in the laboratory. Nature does not always produces all the possible range of Hematite and corundum share exactly the same composition. type of structure. • In the first case, Fe3+ is the cation, in the One can cook up in the lab forsterite, fayalite and other, Al 3+ . their intermediates: • Their coordination number is 6 in each Melt MgO + SiO2 + FeO and let it cool slowly... structure. Yet, in nature, all intermediate compositions are According to your table of ionic radii: rare… Most olivine is close to forsterite because Fe3+ : 0.64 (VI) Fe is taken up by other minerals which precipitate Al 3+ : 0.51 (VI) from the silicate magma as it cools down. Is any solid solution expected? To answer, see if the radii differ by more than 15% or 30%. Corundum (Al2O3) and hematite (Fe2O3) share Polyhedra that share edges are commonly less the same structure. tolerant of substitution than polyhedra that share corners. Note the edge sharing and face Nature of bonding is also different for Al-O sharing among and Fe-O… Unpaired electrons are present in MeO6 octahedra. dorbitals, in ions of Fe (and many other transition metals), but absent in Al3+ . This limits the flexibility of the Despite a larger radius difference, Al3+ and structure towards Si4+ substitute for each other in the substitution among tetrahedral polyhedra of several ions of different tectosilicates (e.g., the feldspars). These radii. tetrahedra share only corners. When a substitution takes place, in a mineral Any substitution can be expressed as an equation: structure, electrical neutrality must be maintained. In the case of Na Al Si3O8 -CaAl2Si2O8 In feldspars, this is illustrated by members Na + +Si4+ = CaNa 2+ + ++Si Al 3+4+ = Ca 2+ + Al 3+ of the plagioclase series: Na Al Si3O8 CaAl2Si2O8 In some high-temperature polymorphs of SiO2, Na + is replaced by Ca 2+ small amounts of Al3+ and Na+ can be found. Si 4+ is replaced by Al 3+ Si 4+ = Na + + Al 3+ When three or more ions are involved in the substitution to preserve electrical neutrality, Which ion, Na+ or Al3+, is substituting for Si 4+ ? we deal with a coupled substitution. (Hint: Which one is closest in size and charge? Where does the extra ion go, in the quartz structure? Interstitial substitution: - an extra ion must be added in the structure to respect charge balance: A x = B y + C z There are many polymorphs of SiO2, stable at different ranges of temperature and pressure. Those This type of substitution occurs in very small stable at highest temperature have the lowest amounts in any structure, and is limited to density. Their SiO4 groups form networks with the structure with channels or cages. most open space. They are more likely to accept impurities (small amounts of Al3+ and Na+ for Si+4). Aquamarine (blue) : Fe2+ Be Al Si O (beryl) has space within channels 3 2 6 18 Heliodor (golden) : Fe3+ formed by the Si O rings. The charge balance 6 18 Green beryl : mixtures of Fe2+ and Fe3+ can be met by a coupled substitution which introduces new ions within the channels. Morganite : Mn 2+ is pink Red beryl : Mn3+ is red Emerald: Cr3+ is … emerald green! Where do these ions go in the structure of iv vi Be3 Al2 Si6O18 ? Why are they often accompanied by Li+, Na+ or K+? Omission substitution: charge balance requires Where does the other ion go? that a site normally occupied by an ion becomes empty (vacant). Some minerals have more space than other, in their structure, to accommodate the extra A x + B y = C z + [empty site] ions involved in a coupled substitution. The structure and formula of pyrrhotite) are The high-temperature polymorphs of quartz, related to those of troilite, FeS, by omission minerals like cristobalite and tridymite, are substitution: less pure than quartz formed at lower temperature. Troilite Fe 2+ S 2- (found mostly in meteorites) On Earth, even low oxygen levels oxidize some Fe 2+ Cyclosilicates, like beryl, contain large to Fe 3+ . As pyrrhotite forms, this happens: channels and can accept large impurities. 3 Fe 2+ = 2 Fe 3+ + [empty site] Overall, pyrrhotite is Fe 1-x S because Are we more Ion Radius Radius likely to see 2+ 3+ 2- (Fe ) 1-3x (Fe ) 2x [ ] x S C.N. = 4 C.N. = 8 substitution + among ions Li 0.74 0.92 Charge balance is satisfied: located in the + positive charges 2*(1-3x)+3*(2x) = 2 negative same family, Na 1.02 1.18 charges within the K+ 1.38 1.51 Note: the value of “x” (proportion of empty periodic table? sites per formula unit) in natural pyrrhotite is Rb+ 1.52 1.61 quite small.
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