December 18, 2001 name: ____________________GS 231: Mineralogy Answers to Final Exam [additional commentary in square brackets]1. (20 points) Island Silicatesa. Give the formulae and names of the common Mg and Fe2+ end-member species of olivines andgarnets in the table below.element olivine formula garnet formulaMg forsterite Mg2SiO4pyrope Mg3Al2Si3O12Fe2+fayalite Fe2SiO4almandine Fe3Al2Si3O12[Listing terms such as Fe olivine or Mg garnet is not an acceptable answer.]b. Given the binary melting diagram for the Fe - Mg olivines, what can be concluded about the degree ofsolid solution and why?The binary melting diagram for forsterite-fayalite shows a two-phase field (liquid+solid) betweenthe solidus and liquidus, with fayalite melting at much lower temperature than forsterite. Thelack of a solvus on the diagram indicates that there is continuous solid solution at hightemperature between forsterite and fayalite. [There have been an unconfirmed suggestion of unmixing at relatively low T, < 500°C, where it isvery difficult to approach equilibrium in a dry system both in nature and the laboratory.]c. An unusual olivine, monticellite, is known with a formula involving one mole of Ca and one mole ofMg. Given other mineral systems with similar chemistry, is it likely that monticellite may be found withMg olivine or not? Give a brief explanation of your conclusion.Monticellite is an olivine with the formula CaMgSiO4, compared with forsterite, Mg2SiO4. Othermineral systems with similar cations include diopside (CaMgSi2O6) – enstatite (MgSiO3) anddolomite (CaMgC2O6) – magnesite (MgCO3). These systems have a wide solvus between the twophases, suggesting that monticellite and forsterite have similar relations. [The olivine quadrilateral closely resembles the pyroxene quadrilateral, with continuous solutionbetween monticellite and kirschsteinite (CaFeSiO4) and of course continuous solution betweenforsterite and fayalite, but with limited solution between the calcian and ferromagnesianolivines.]d. A surprising find is the occurrence of a fully oxidized olivine called laihunite. Without knowing aboutlaihunite, which has been found rarely in altered and oxidized iron formations metamorphosed at high P-T, provide a possible mechanism by which ferric iron may substitute in olivine.The problem is in identifying a charge balance mechanism that will allow Fe3+ to substitute forFe2+ in the VI sites of olivine. One may consider exchanges such as NaFe3+ = 2Fe2+, Fe3+Al =Fe2+Si, 2Fe3+ = Fe2+Si, and the vacancy-related substitution [ ](Fe3+)2 = 3Fe2+. These lead to therespective end-members NaFe3+SiO4, Fe2+Fe3+AlO4, Fe3O4 and [ ](Fe3+)2(SiO4)3. The first is notknown as a mineral, the second is chemically equivalent to a magnetite-hercynite solid solution,and the third is magnetite, and the last is the major component involved in laihunite.[I accepted any of these answers. Laihunite has magnetite solid solution as well as the vacancysubstitution. It forms from fayalite by reaction with highly oxidized late-stage fluids.]2. (20 points) Chain Silicatesa. (5 pts) List the different types of chain silicates based on fundamental differences in their basicstructures.single chain silicates: [pyroxenes and pyroxenoids]double chain silicates: [amphiboles]triple chain, complex: [pyriboles, biopyriboles]b. (5 pts) The chemical classification of the main pyroxene and amphibole groups is similar. Withoutlisting individual mineral names and formulae, Give four major chemical series that apply to both groups.sodic [jadeite, aegirine; glaucophane, riebeckite, arfvedsonite]calcic [diopside, hedenbergite, augite; actinolite, hornblende]ferromagnesian [hypersthene, pigeonite; anthophyllite, cummingtonite]sodic-calcic [omphacite; pargasite, hastingsite, richterite]“lithic” [spodumene; very rare lithian amphiboles]c. (10 pts) Sketch the pyroxene quadrilateral in its standard form and label the corners. What pyroxeneseries lie within the quadrilateral rather than along or very close to the edges? Show the areas withexpected solid solutions with light shading and indicate where solvi are found with tie lines between thecoexisting pyroxenes.cf. Figs. 14.2a, 14.6 in NesseThe corners of the quadrilateral are diopside, hedenbergite, enstatite and ferrosilite.The pyroxenes inside the quadrilateral are augite and pigeonite. [Few bothered to answer this.]The major solid solutions are diopside-hedenbergite-augite, pigeonite, and enstatite-ferrosilite.The solvi lie between each of these series.The tielines between the respective solid solutions form fans that are more or less parallel to theleft and right hand sides, and sweeping between these.3. (15 points) Sheet SilicatesThe common sheet silicates may be distinguished by their basal spacings and whether they aredioctahedral or trioctahedral.a. Fill in the table below with the names of the common Mg-rich and Al-rich sheet silicates. If nocommon representative is known, put a dash in that section of the table. For each group indicate thenature of the tetrahedral and octahedral layers in shorthand (i.e., TO structure).basal spacing dioctahedral trioctahedral T&O bonds7 Å kaolinite serpentine TO9.3 Å pyrophyllite talc TOT10 Å muscovite phlogopite TOT + K14 Å  chlorite TOT + Ob. Chlorite is a hydrated magnesium aluminum silicate. What mineral assemblage might it react to formin the absence of quartz?chlorite = olivine + enstatite + spinel + H2O orchlorite = olivine + enstatite + pyrope + H2O[reactions given in class notes]c. Pyrophyllite is a hydrated aluminum silicate. What mineral assemblage involving one or more indexminerals might it react to form? Balance the exact reaction(s).pyrophyllite = kyanite/andalusite/sillimanite + quartz + H2OAl2Si4O10(OH)2 = Al2SiO5 + 3SiO2 + H2O[The reaction was given in class notes. The use of index mineral should have triggeredconsideration of Al2SiO5 minerals, and the reaction is then simple to balance if you know theformula of pyrophyllite.]4. (20 points) GeochronometersThe minerals zircon, monazite, and xenotime are common accessory minerals in a wide variety ofcrustal settings, ranging from concentrates in sand deposits, to metapelitic and granitic rocks.a. Give the formulae for zircon, monazite and xenotime.zircon ZrSiO4, monazite CePO4, xenotime