W. H. Slabaugh Oregon State College Corvallis
Crystal Models
W i t h the recent trend towird the st,ructural npproarh ill tewhing general chemistry the 11eed for large, ncnrly pwfect, crystal? has berornr evident. Sinre the cost of surh crystals n.ouli1 he prohibitive, the ;iuthor turned t,o the corlst,nwtio~~ of crystal models. Several 'riteria for such visual uids :ue evident.: These rrpstt~l models should be hrge enough and strong enongh to wit.hst,and classroom use, both a t the lert.1u.e t,:~hleand by t,he indil-idual st~tdeut. I'urt,hermorr, it is desirable that they he suffi(tie11tly i m w a t e to permit malysis and interprct:~tio~rof the rrystd st~ructures. In order t o meet t,his ilred, a set of modrls \\-;IS co~~structed from about $25 worth of P l ~ x i g l oht:~i~lecI :~~ through :i commnr:iill supply house for plastic.;. 'I'll? KI\V stork obtained for this purpose c011sistcd of hnth one-i~~ch and t.wo-i~~(th thirk shert Plt.xigl:w. The models s h o n . ~in~the tigure w r r sketched ortt O I I thr plastic sheet and \\-err rr~ughlyshtrped hy ruttiug the plast,ir wit11 an eight-i~whh e ~ ~ rsaxT-. ll This pmnitted the construotio~~ of nll of t,he hasir crystal Torma \!-l~ir.h w e usually encon~~trred in a rourse of general ch~mist,ry. Where sm;rll fanes e r e involved, these could hr r e d i l y sh:~ped hy g r i n d i ~ ~the g pkistir 011 an o r d i ~ ~ n r p c::~rhorondumwherl.
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1 ,I Crystal models m a d e of Pleriglor. Upper row: simple cube, octohedrol f m e d cube, oct.hedron, dodecahedron. Lower row co1cite. .rrogonite. octagond p r i m . and quartz l h e i o g o n d prim).
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Journol of Chemical Education
Alter the rough shapes were ohtained, thc most critical operation \\-as polishi~lg, arhievrd as follo\vs: I'irst, t,hc rough OIIW were sanded down or1 s i l i r o ~ ~ rxrhidr No. 80 p p e r unt,il the nu~jorirregularities \\-err removrd. The p:~per mils placed 011 a flat s ~ ~ r f a c e . The pl;tst,ic, held in OIIP I l a ~ ~ dw:ls , moved back and forth on t,he s:lnd paper in order t,o p r o d ~ ~ c:Ie flut snrf:~~:e.Next,, the pl;~stic\ws fi~~ished up n.ibh So. 400 Wet,ordry p n p e ~ t i r s tdry 1111ilt,hen wet. 'I'he surf:we of the plastic :it this point. was quite smooth 1)ut 11ot opt,irally clear. -4s:I Inst st,ep, t.he plnst,ic was buffed with a ranvas-r.overed disc: tre;lted first witl~white diamoud hnffi~~g cwmpo~md grade XSX, :u~d then fi~~ished OR wit11 jm\-elera rnugr. 'I'lle resrlltiug model \\-:IS gloss-clear, gron~etrir:rlly ;icrru.:rtr to the extent. required for the prrsellt purpose, ; I I I ~ qnitr rrsist.:~~~t, to the scl.at,ches : I I ~:~hrnsio~w r n r n u ~ ~ t r wind n r d i ~ ~ a r y use. -4s a p r e r ; ~ u t i o ~ho\\-ever, ~, the plastic al1011ld be treated :ts if it, were glass insofar 11sits frapilr 11:ltnrc i~ co~rcrrnrd,because rorllers cnn he chipped by dropping or hy uhrnpt shock. However, the plast,ic models are ~ o ~ ~ s i d e r : ~more hly r w i s t i ~ ~ to~ht ~ ~ i ~ lt ih~ag~nrdim~ry e~ glass. It. is est,imat,ed t l i ; ~ tabont :40 hours of studrnt Iahor were required t,o make the eight crystal form-; ~ I I O V I I in the figure. This s t ~ ~ d ehad n t I I O prcvious exprrie~~ce but,, with a rni~~imum :llnolmt of guid;mce, n o h i r \ d t,he tech~ique I I P C ~ S S ~ I ~to ? p r o d ~ c e rather attfi~ntiw ~nodrls. I n add~t,io~r to the h:wic rrystal forms, s m ~ r : ~ l rryst:~lsreprese~~t:~t,ive of urtunl materials were prepared ill order t,hnt some of t,hrir 1111osu;r1 rharacteristirs could be ill~lst,ratrd. I'or e x ~ n.~ l i, lone e , mnv identify in the figure a rrystal of qu:~rte,;I simple cub;, i ~ n da diiimond. The q ~ u r t , zmodel was made lerge twough t o sllo~!- :dl t,hr faces involved ill t,his p;trt,icr~l:~r crystal, i11r111di11g the small faces whirh i ~ ~ d i o ;either ~ t e ;r right,-ha~~ded or :I left,-handed qu:~rtz rryst;~l. Sillre only thr larger quart,z crystals actually rho\\; t,hese left,. urrd righthanded faces, i t is of ro~~sider:~ble interest to the student to rx;lrnine t.he models : I I I ~ i~rriveat. t,he co~rrlusio~rs b:wd up011 his own observations. Models of :I pair of nalrium carhonak crvst,:~lsin the forms of calcite nud aragonite are fou~ldio he (luite inst,ructive when thrir actual shapes are rompnred to t,lir int,ernal rnolrr.~~I;rr arral~gementof the rryst:~lsthemselves.
