chemical principles exemplified
ROBERT C. PLUMB Worcester Polytechnic Imtifvte Worcerter. Molsochusdh 01609
Quartz Geodes Illustrating principles of solubility and the chemistry o f silico
Information provided by Professor Konrad R. Krauskopj, Stanford University Have you cver marvelled a t the beauty of quartz geodes in a mineralogy collection-thc large, delicately colored, sparkling crystals of quartz which have been synthesized by a geochemical process? No one k n o m xith certainty how nature accomplishrs this marvelous feat, but sufficient facts' are available that a reasonable mechanism may be proposed, and this mechanism brings out some principles of solubility that students
Sio,.XH~o(s)
+ 2H10(1)
=
H&SiO,(aq)
+ XH-0
(1)
The only soluble silicic acid species which is important to the problrm before us is monosilicic acid, H4Si04(aq). It is not significantly ionized until the pH is greater than 9, and it can be polymerized only if it is not a t equilibrium xith a solid phase, conditions with which we shall not be concerned. Howcver, we are concerned with n variety of solid silica forms. These include quartz, rhich is the thermodynamically stable modification of SiOz a t room temperature, amorphous silica (anhydrous), silica gel, and colloidal silica. The value of K,, for the solubility equilibrium depends upon which solid species onr has in contact with the solution. Amorphous silica, silica gel, and colloidal silica arc metastable forms of silica; they behave as
though they are distinct chemical species. The solubility of silicic acid in contact rvith quartz is about. 7.3 X 10W5 M a t 20°C and 1 atm.' Thr solubilities of silicic acid in contact xith amorphous silica, M. silica gel, and colloidal silica are all about 1.8 X The information above leads one directly to a mechanism by xhich quartz crystals can grov. Water circulated through a bed of amorphous silica or silica gel vill bccomc saturated xvith rrsprct to t h r solid form of silica present. That ~ v a t r ris supersaturated with rcspcct to a quartz crystal and if it is circnlatrd over such a crystal the quartz crystal d l grov. It is thought that gcodc gro~vthgrts startrd by nucleation of quartz crystals on the surfacr of a shrll or an open cavity in a sedimentary deposit. If the sediment contains amorphous silica, then t h r m t c r circulating through it will feed the grolvth of the original crystals and cvcntually macroscopic crystals d l rrsult. H o v long will it takc? It has rccently brrn drmonstrat,edZthat minute quartz crystals can grow from ground quartz crystals in contact with x a t r r in three years a t room temperature. I n that expmimcnt thc ground quartz v a s of a much largrr particle size than amorphous silica, and the solution x i s only slightly supersaturated with respect to bulk quartz crystals. As we saw abovr, nature could provide conditions n.h-here thc supersaturation is much largrr and morlX rapid growth might be possihlr. 1 KHAUSROPF, K. B., i n "Silicain Sediments" (Edilor:I H ~ ~ I L A X D , IT. A,,), Society of Economic Psleontologists and llinel.nlogiits, Tulsa, Okln., 1959, Chspler 2. 1 Mnca~.xzm, F. T., .AND 11. Gws, Scirner, 173, 533 (1971).
Volume 49, Number 1 I , November 1972
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