November 21, 2002 name: ___________________GS 231: Mineralogy Second Midterm Exam1. (30 points) Sulfide Mineralsa. Give the formulae and names of a common sulfide mineral for each designated metal ion below.element formula nameCuZnPbb. The common sulfides of iron are pyrite and pyrrhotite. Briefly explain from a crystal-chemicalviewpoint why pyrite is brittle and hard whereas pyrrhotite is more malleable and is softer.c. There are two polymorphs of CdS, hayleyite and greenockite. Although rare, each is found in a widerange of rocks in nature. Greenockite is hexagonal and hayleyite is cubic. Thermodynamic data at STPfor these minerals are as follows:phase entropy volumehayleyite 70.00 J/mol K 2.9932 J/mol bargreenockite 70.29 J/mol K 2.9934 J/mol bari. Assuming that each polymorph has a stability field in P-T space, which phase is stable athigher pressure? Why? ii. Which is stable at higher temperature? Why?iii. What can be estimated about the nature of phase diagram for the two polymorphs with theuse of the data above?2. (40 points) Garnetsa. Give the formulas and names of four natural end-member garnets.b. What is a name for the very general silicate grouping to which both garnet and olivine belong?c. What is the degree of polymerization of silica tetrahedra in this group? Briefly explain why this is so.d. Manganese is one of the few elements with three valence states, Mn2+, Mn3+ and Mn4+, commonlyattained in the Earth’s crust. If a hypothetical synthetic garnet is made only with Mn in these threevalence states and oxygen, what would its formula be? Which valence would substitute in which of thecation sites in garnet?3. (30 points) New MineralsOne of the postdocs in Rod Ewing’s research group is interested in weathering products ofradioactive minerals, and has been looking at a pyrochlore sample from Bancroft with theSEM. He used EDS and BSE to determine that a mineral which forms in veins and fractures inthe pyrochlore contains primarily Si, Pb and Fe. The two possible mineral matches he foundwere melanotekite, Pb2Fe3+2Si2O9, and jagoite, Pb3Fe3+Si4O12(OH), but the cation ratios fromsemiquantitative analysis did not match either of the two minerals. He suspects he might havea new mineral.a. What are the two aspects that our intrepid researcher must define in order to characterize itas a new mineral?b. What laboratory techniques should he apply?c. Briefly describe the way these two machines