Geochemistry for Chemists John D. Hosteffler San Jose State Universi'
,. San Jose. CA 95192
Theabsenceof geochemistry fromour chemistry curricula seems inex~licahleonthe hasis of the maturitv ofthesubiect or its re1e;ance to the rest of chemistry. I& absence may simply he amatter of tradition and imorance. The traditional geochemistry course has been taught by and for geologists. Geology departments usually offer such courses as upper division or graduate electives with mineralogy and petrology prerequisites which in turn have a physical geology prerequisite. Thus, geochemistry has been buried in the geology curriculum, and chemists have remained relatively ignorant of its subiect matter. This a k c l e describes a geochemistry course for chemists. Some eeneral course information is urovided. and then its subject matter is outlined topic by topic. he concluding section considers the uses of geochemistry in a chemistry curriculum including geochemical "real world" examples ( I ) , geochemistry in general chemistry, and geochemistry as an elective. Geochemistry (Chem 421 and 422) was offered as a two-
semester upper division elective by the Chemistry Department of the University of Colorado a t Colorado Springs (UCCS) in 1982183 and also in 1983184. Both courses were taught as three-credit lectures. The only prerequisite for Chem 421 was general chemistry. Chem 421 served as prerequisite for Chem 422. The course was desiened to anneal " .. to chemistrv and eeology majors and to local high school science teachers. Most of the students actually taking the course were chemistry majors, but geologists and science teachers were also represented. The ~articioationof eeoloeists in the eeochemistrv course u,o;tld undoul,ted~~ be greater at an i n s t ~ u t i o nwhich offers a reolorv maior ( U C W do~.snot). Enrollments varied from 5-i0 studentiper semester. Three geochemistry texts (2-4) have recently been published. Each of these texts have their individual merits and weaknesses, but no one was judged sufficient for our course. Students were asked to nurchase one of these texts (Brownlow's in 1982183 and ~ r a u s k o p f in s 1983/84), and the other texts were made available in the lihrarv on reserve. The format of the course consisted of assigned readings, w
Primary m a t e d Figwe 1. The rack cycle. Reservoirs tor primary material entering Me cycle from the earth's interior are shown as rectangles. Geological processes which carry rock material from one reservoir to another are indicated by arrows. Diagenesls includes all changes which occur as sediment becomes rock including compaction. cementation, recrystallization, and replacement. Anatexis refers to the melting of rock to form a magma (Fig. 12.4, fmm "Principler,of Geochemistry" by 6. Mason and C. 8. Moore. 4th ed.. 1982. Reprinted by permission of John Wiley and Sons.Inc.)
Volume 62
Number 10 October 1985
,823
Table 1. Syllabus of the Geochernlstry for Chernlsts Course
arranged according to three major themes: 1) minerals and rocks, 2) the formation of the earth, and 3) the rock cycle. The minerals and rocks topics provided the mineralogy and petrology needed to understand the rest of the course. The formation of the earth included the origin of the earth's elements, the formation of the solar system, and the suhsequent differentiation of the earth into core, mantle, crust, atmosphere, hydrosphere, and biosphere. The rock cycle topics dealt with the geological processes which produce igneous, sedimentary, and metamorphic rocks on the earth as i t now exists (see Fig. 1). The chemical topics-crystal chemistry, nuclear chemistry, thermodynamics, aqueous and organic chemistry-appeared a t the appropriate point in the course to support related geological topics.
I. Minerals and Racks Crvstal chemistrv: comoosition and structure of crvstals - ,-~ - Systemat c monera ogy: e assificatlan and description of Common minerals ntrOdUCtm to rocks. thtl rock cycle. class tical on and description of common rocks 11. F m t i o n of Me Earth ~~
.
~~~~~
Nuclear chemistrv: nuclear stabllitv and nuclear reactions ~ - -- isotope geocnemootry: n~clsarclocks, wacer Isotopes E emental aoLndanceo. in the earth, metwrotes, and me dn.vase Orig n of the elements: the Big Bang, stars and nlrc wsymhssir The solar system: description and evolulion Differentiationof the earth: into care, mantle, crust, amsphere, hydm Sphere and biosphere Ill. The Rock Cycle ~
~
~~
~~
Thermodynamics: principles, calculations, and phase diagrams Magmatic processes: formation of magma, Igneous rocks, hydrothermal fluids, and volcanic gases Water chemisby: acid base, redox and solubility equilibria In the hydrologic cycle Chemical weatherinn: reactions formino -clavs. . soils. and natural waters Sedimentation: formation of detrital, biogenic and chemical sedimenb Organic geochemistry: natural organics, toss11 fuel genesis, and the carbon cycle Economic mineral deposits: genesis of deposits, occurrence of the elements
lecture notes, transparencies, problem sets, and exams. Lectures incorporated material from a wide variety of sources. These sources were referenced in a bibliography (2-50) which was made available to students to facilitate further research. The pictorial nature of geochemistry made transparencies a very desirable and important feature of the course. The problem sets were designed to challenge students' understanding of the readings, lectures, and transparencies and also to serve as practice exams. Library exercises adapted from the CLEAR materials (51)were used to introduce students to the geochemical literature. Students were required to research this literature on a geochemical topic of their own choosing and to present the results in oral and written form. The content of the course was divided into chemical and geological topics (see Tahle 1).The geological topics were
Mlnerals and Rocks Crystal Chemistry (2- 12)
Crystal chemistry began with a review of the comuositions and modes of bonding b f substances forming crystals. Five types of crystals were then distinguished on the basis of the particles present and the bonding between the particles: metallic, atomic, molecular, covalent (network), and ionic ( n e t w r k ) crystals. The characteristic physical properties (luster, melting point, density, hardness, malleability, conductivity) as.ioriated with each crystal type were described and related to the crystal composition and structure. Descriptions of crystal structures inevitablv involve some crysrallugraphic concepts. Our course made use of unit cells, Rravais lattices, Miller indices, symrnetrv elements and nuerations, and ~ermann-Mauguinsymbofs. No group the& and no X-ray diffraction was included. Some information on crystal morphology was provided so that students might perceive the correlation between the external form or habit of a crystal and its internal symmetry. The crystal chemistry topic concluded with a consideration of the compositions and crystal structures of 21 nonsilicate minerals (see Tahle 2). Models were distributed to small groups of students, and they were asked to determine coordination numbers, find and classify unit cells, count formula units per unit cell, and draw floor plans. Models of metallic crystals were examined first and then models of
Table 2. Cornposltlons and Structures of Nonslllcate Mlnerals Crystal Type
Particle Packing* ccp atoms bcc atoms hcp atoms
Metallic
Molecular CCP
m~lecule~
Covalent
Mineralb gold a-iron lridwmined a-sulfur Ice i methane diamond graphite
Z,Formula, CNsC 4AuXt1 2Fev'l1 2(lr.O~)~" 18Sa 4(H# 4(CH4IX" 8ClV 4C"'
Unit Cell cubic cubic hexagonal orthorhomblc hexagonal cubic cubic hexagonal
lonid -ccp anions -ccp anions -ccp anions -hcp anions -hcp anions -hcp anions -hcp anions - h ~ p anions -simple cubic anions -simple cubic anions
'The symbol
-
halite Sphalerite spinel nlccolite brucite gibbsite hematite wurtzite C5CIe fluorite rutlle calcite aragonite
cubic cubic cubic hexagonal hexagonal monoclinic hexagonal hexagonal Cubic Cubic tetragonal hexaqonal orthorhombic
denotes approximately. bThe lawen-temperahlrepolymwph is dealgnsfed o,the nen lowest 8, stc.
