W. Frank Kinard The College of Charleston

Charleston, SC 29401 I The Chemistry of Seashells

Marine organisms that utilize calcium carhonate for the ronstruction,f their skeletal material build these structures from two different crystal forms. aragonite and calcite. Shells composed of. calrite are further suhiividerl into low magne- sium and high magnesium calcite with [he dividine, line at approximateiy 4 mole percent magnesium carbonate.

Since the ratio of magnesium to calcium in seawater is about 5 to 1, the fact that most marine organisms are able to dis- criminate against magnesium in favor of calcium is of great chemical interest. The magnesium content of shells is pri- marily controlled by the mineralogical form of the precipitate since aragonitic forms rarely contain over 1 mole % MgC03 while calcitic skeletons rarely contain less than 1% MgC03 and mav he as much as 20t30% maenesium carhonate. Other factors which have been cited as Gfluencing the magnesium content of shells are the water tem~erature and the Dhvloge- netic level of the organism. ~ c c o r d i n ~ to Chave, ti&: isan apparent correlation between the amount of magnesium in- corporated into the shell and the temperature of the water in which the organism lives (I). That is to say that tropical forms tend to contain more magnesium than a similar species living in more northern waters. Also, the phylogenetic level of the organism appears to play a role sinreanimals which are higher on the evolutimary scale apparently can disrriminate against magnesium to a greater degree than more simple organ- isms.

An interesting aspect of the carhonate mineralogy is the relative stability of calcite as compared to aragonite. Under conditions that are prevalent on the surface of the earth (1 atm of pressure, 298 O K ) , aragonite is unstable with respect to recrystallization to calcite. In addition, high magnesium calcite is also unstable and will revert to low magnesium calcite in the geologic record. This recrystallization can be examined by comparing the crystal structures of fossil species with closely related species living today.

The role of magnesium ions in the transformation of ara- gonite to calcite has been the subject of a large body of geo- chemical literature. Basically, the problem has been ap- proached as being either a dissolution-recrystallization process or a solid-state transformation. The fact that the fine struc- tures of many fossils are preserved in the geologic record in- dicates that the solid state nrocess certainlv occurs. In both processes, magnesium ions Ean inhibit the tiansformation of aragonite to calcite (2,3).

We have used infrared and atomic absorption spectrometry in an introductory chemical oceanography course to introduce students to carhonate mineralogy by having them determine both the crystal structure and the magnesium content of shells that they have collected. The use of infrared spectral analysis to determine crystal form provides a low cost method of in- troducing studen& cn crystal structure, especially where X-ray equipment is nm readily available.

Infrared Studles The determination of the crystal form of a mineral normally

requires a polarizing microscope or X-ray diffraction unit. However, in the case of calcium carbonate minerals, the dif- ference in the site symmetry of the carhonate anion in ara- gonite and calcite creates a difference in the vibrational spectra providing a basis for the infrared determination of

INFRARED SPECTRA OF CARBONATE MINERALS

I200 1000 800 6(

WAVENL

)O IME

i i I I IMO 4000 800 600

IER Icm-'I

Inhared spectra ol seashells illushating lhe aragonite and calcite struchves of calcium carbonate.

Table 1. Carbonate Infrared Spectral Assignments

"1 "2 v3 u4 Calcite 876 s. sh 1428 s. b 710 r, sh

850 w Aragonite 1083 m, sh 857 s, sh 1475 m, b 710 r, sh

840 w 695 s. sh

Table 2. Shell Chemistry ol Some Common Marine Organisms

Mole Common Percent

Family Examples M&@ Min alogy

Gastropods conchs, whelks 0.1: 2.4 aragonite or a minure of calcite and aragonite

Pebypods oysters, clams 0.1- 2.8 aragonite or calcite or a mixhre 01 calcite and aragonite

Echinoids sea urchins, 5.5-16 calcite Sand dollars

Barnacles t.3- 4.6 calcite Decapod crabs 1.7-23 calcite

Crustaceans

mineralogical form. The infrared spectral assignments re- ported by Chester and Elderfield ate given in Table 1 (4). We have used the unique aragonite band at 1083 cm -1 to auali- tatively differentiate bet&n aragonite and calcite. compere and Hates ha\,e reported that for fresh water mollusk shells. quantitative analisis by infrared spectrometry permits an accuracy of crystal structure determination approaching that . . obtained hy X-ray analyses (5).

- The experimental procedure involves grinding a shell

fragment weighing slightly more than 1 g in a mortar. The sample is screened through a 60 mesh wire screen until about

Volume 57. Number 11, November 1980 / 783

100 mg of sized sample is obtained. We have found that better spectra are ohtained if the particle distribution is more uni- form than simple grinding produces. Also, while Compere and Bates made KBr el lets for analvsis. we have found that mineral oil mulls are adequaw for qualitative determinations. The remaining portion of t he ground sample is saved for the magnesium determination. The carhonate sample is then mulled with mineral oil and the spectrum recorded on a Per- kin-Elmer Model 267 Infrared ~pectrometer hetween 1200 and 600 em-'. Figure 1 illustrates some student results for a fresh clam shell and a fossilized oyster shell. Note the sharp band characteristic of aragonite in the spectrum of the clam shell.

Magnesium Analysis The maanesium content of the shells was determined by

atomic ahLorption spectrometry using Perkin-Elmer odd 103 or Model 460 spectrometers. The sample was prepared by dissolving 1 g of ;he ground shell in a minimal volume of concentrated hydrochloric arid and diluting to 100 ml. De- pending upon the sample and its origin, n small amount of insoluble material may he encountered as a result of incor- poration of sand or clay in the shell. Freshly collected speci-

mens may also have a small amount of organic material re- maining in the shell. When they were encountered, these in- clusions did nut interfere with the overall analysis.

Since the solut~ons contained a large excess of calcium, all standard solutions were prepared by pipetting an appropriate amount of marnesium standard into a volumetric flask and diluting with a stock solution containing 1.0 g of dissolved calcium carhonate Der 100 ml. Maenesium concentrations were determined h y a standard cur& method. Samples con- taining high levels of magnesium were diluted and appropriate corrections for dilution were applied. Table 2 presents some data given hv Chave for a wide varietv of marine oraanisms (6). Student results agreed quite well with this data except for some echinoderm samples which gave values about 5+10% higher than the values reported.

Literature Cited

784 1 Journal of Chemical Education