GS 231: MineralogyFinal Exam AnswersSi is a tetrahedrally coordinated (IV) siteDecember 16, 2002 name: ___________________GS 231: Mineralogy Final Exam Answers[Information in square brackets is extra commentary]1. (50 points) Oxides and hydroxidesa. (10 points) Name two common iron oxides and one common iron hydroxide, and give their formulae.magnetite Fe3O4hematite Fe2O3goethite FeOOHb. (10 points) Briefly explain why gibbsite, Al(OH)3, has a much smaller range of stability in P-T space thandoes diaspore, AlO(OH).The mineral gibbsite has three times as much hydroxl as diaspore. The tendency is for gibbsite todehydrate to diaspore + water at quite low temperatures (0-50°C). With only one mole of OH,diaspore is more stable. [It dehydrates to corundum + fluid at moderate temperatures, 400-500°C.]Hydrates are often stable at low temperature, but with increasing temperatures, the formation of H2Oliquid/gas, with its greater vibrational capability and higher entropy, is increasingly favored.c. (10 points) Rutile is the most common Ti-oxide. It occurs in low-grade calcitic marbles with quartz butsphene (titanite) is found instead above about 500°C. Balance a reaction that explains these relations.rutile + calcite + quartz = sphene + CO2TiO2 + CaCO3 + SiO2 = CaTiSiO5 + CO2d. (10 points) Why is it that quartz and other silica polymorphs are classified as silicates rather than oxidesin most mineralogical texts?Although the formula clearly qualifies SiO2 polymorphs as oxides, they have 100% polymerization ofthe silica tetrahedra, possess structures quite similar to other framework silicates such asnepheline, and thus are structurally related to framework silicates. Most oxides have octahedral andlarger coordinated sites.[The spinels are a good example of an oxide group with IV as well as VI sites, although they take upvery limited Si in their structures except at very high pressures. By the way, the formation of oxidesdoes not require the process of oxidation to occur. They form like silicates in rock systems thathave some available oxygen bound with Si and other cations.]e. (10 points) Why is it that stishovite, the high pressure polymorph of silica, might nonetheless beclassified as an oxide rather than a silicate in these same texts?Stishovite has the rutile structure with Si in VI coordination. Its structure is identical to an oxideand therefore by the same logic as above could be classified with oxides.2. (45 points) Pyroxenes and Olivinesa. (10 points) What is the basic structural arrangement of pyroxenes and what is that of olivines? Give ageneral formula for each group and indicate all cation sites in terms of their coordination number.Pyroxenes are single chain silicates with an general formula of RSiO3 or M2M1Si2O6Olivines are island silicates with an general formula of R2SiO4 or M2M1SiO4where M2 is a larger, more distorted, octahedrally coordinated site (VI) [Nesse gives as VIII in cpx]M1 is a smaller, less distorted, octahedrally coordinated site (VI)Si is a tetrahedrally coordinated (IV) siteb. (15 points) Forsterite is not found with quartz although enstatite may occur with quartz. On the otherhand, fayalite is found with quartz in iron-rich granites, and ferrosilite does not occur there. Ferrosilite hasbeen found, however, in high-pressure BIFs. Explain these equilibrium associations with two chemicalreactions in the pure MgO-SiO2 and in the FeO-SiO2 systems and balance the reactions.olivine + quartz = orthopyroxeneMg2SiO4 + SiO2  2MgSiO3Fe2SiO4 + SiO2  2FeSiO3The Mg reaction proceeds to the right at all P-T, so forsterite is never stable with quartz.The Fe reaction proceeds to the left at low pressures and to the right at high pressures, soferrosilite is only stable at high P. [The latter reaction is located at ca. 10-15 kbar, 800-1200°C][Neither MgO nor FeO was mentioned as a phase, so they shouldn’t be used in the reactions.]c. (20 points) Several rare nonsilicate minerals have an olivine structure, M2M1SiO4. These include aphosphate with Li and Mn (lithiophilite), an aluminate with Be (chrysoberyl), a borate with Mg and Al(sinhalite), and a sulfate with potassium (arcanite). For each mineral, identify the cation site in the olivineoccupied by each element along with its valence, and describe the exchange that relates that mineral withforsterite. More than one element may occupy the M sites. Fayalite is shown as an example.mineral M2 site valence M1 site valence Si site valence exchangefayalite Fe 2+ Fe 2+ Si 4+ Fe = Mglithiophilite Mn 2+ Li 1+ P 5+ LiMnP = Mg2Sichrysoberyl Al 3+ Al 3+ Be 2+ Al2Be = Mg2Sisinhalite Mg 2+ Al 3+ B 3+ MgAlB = Mg2Siarcanite K 1+ K 1+ S 6+ K2S = Mg2Si3. (60 points) Biotites and HornblendesBiotites and hornblendes are “junk-box” silicates having wide-ranging solid solutions involving many of thesame cations and anions.a. (22 points) Considering the ideal phlogopite formula K2Mg6Si6Al2O20(OH)4, indicate on which sites thefollowing ions may substitute in the structure: O, F, Na, S, Cl, Ca, Ti, Mn, Fe2+, Fe3+, Rb, Ba. If some atomssubstitute on more than one site, list all sites that they may occupy.K Na, Ca, Rb, BaMg Ti, Mn, Fe2+, Fe3+Si Al, rarely Fe3+OH O, F, S, Clb. (18 points) Considering the ideal tremolite formula [ ]Ca2Mg5Si8O22(OH)2, indicate on which sites thefollowing ions may substitute in the structure: O, F, Na, Al, Cl, K, Ti, Cr, Mn, Fe2+, Fe3+. If some atomssubstitute on more than one site, list all sites that they may occupy.[ ] Na, KCa Na, Fe, Mn, Mg [the latter three esp. in ferromagnesian amphiboles]Mg Al, Ti, Cr, Mn, Fe2+, Fe3+Si Al, rarely Fe3+OH O, F, Clc. (20 points) At high pressures, amphiboles tend to react to form a sheet silicate and a pyroxene via solid-solid reactions. Identify the high-pressure products of glaucophane, Na2Mg3Al2Si8O22(OH)2, by name, writetheir formulae, and balance a simple reaction among the three minerals.glaucophane = jadeite + talcNa2Mg3Al2Si8O22(OH)2 = 2NaAlSi2O6 + Mg3Si4O10(OH)2 [This reaction is located at ca. 35 kbar and 700-800°C. Its slope, and locus at lower temperature, is still not well known.][Trying to use enstatite yields 2MgSiO3 + Na2MgAl2Si6O16(OH)2or 3MgSiO3 + Na2Al2Si5O13(OH)2, neither of which is a sheet silicate]4. (60 points) Feldspar GroupThe common