Changes in the Physical and Chemical Properties of Biogenic Silica

18 Changes in the Physical and Chemical Properties of Biogenic Silica from the

Downloaded by CORNELL UNIV on October 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0147.ch018

Central Equatorial Pacific I. Solubility, Specific Surface Area, and Solution Rate Constants of Acid-Cleaned Samples D A V I D C. H U R D and FRITZ T H E Y E R Hawaii Institute of Geophysics, Honolulu, Hawaii 96822

General trends of decreasing solubility of acid-cleaned radiolarians with increasing age suggest that cherts and porcelanites (recrystallized cristobalite and quartz) are presently forming. The thermodynamic properties of biogenic silica are between those of silica gel and cristobalite. The specific surface area of biogenic silica assemblages has decreased by two orders of magnitude in the last 40 million years. Heterogeneous solution rate constants for pure substances yield valuable information regarding the free energy of activation of solution processes. These constants are quite sensitive to contamination from a mixture of various silica forms and may not be as immediately useful as the solubility information. " D iogenically precipitated silica is a metastable silica polymorph which must eventually alter to quartz under the earth's surface conditions. Present observations of deep-sea sediments suggest that this transforma tion may occur directly or through an intermediate, alpha cristobalite. Several models have been proposed to ascertain the rate at which these processes occur. This series of papers tests these models and offers simple but powerful methods for detecting changes i n crystal form as a function of geologic age. As noted i n the title, the first section of this research deals w i t h the changes i n solubility, specific surface area, a n d solution rate constants of the substances studied. Since it is important to understand the extent to which these properties change with changes i n crystal form, w e first 211 Gibb; Analytical Methods in Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

212

ANALYTICAL METHODS IN OCEANOGRAPHY


review previous workers' data on each topic. In addition, we consider the initial properties of biogenically precipitated silica relative to its more stable polymorphs and how these properties change as the transi tions to these polymorphs occur. F r o m the beginning it may be argued that the study of only acid-cleaned materials can hardly be extrapolated to the complex interactions occurring among silica polymorphs, metal oxides, and alumino-silicate minerals i n the sediments. However, we suggest that such reactions alter only the rate and not the final outcome of the silica transitions. Further, if the properties of the starting material relative to the end products are not well understood, how is it possible to understand the extent and nature of these transitions? Solubility of Biogenically Precipitated Silica, Vitreous Silica and Silica Gel, Cristobalite, and Quartz in Aqueous Solutions A number of authors (1, 2, 3, 4, 5) studied the solubility of bio genically precipitated silica. The solubility of artificially precipitated silica (silica gel) and of vitreous silica, two forms of silica which have similar solubilities also have been studied (6-16). Biogenic silica and silica gel probably resemble each other more than either resembles vitre ous silica. Sosman (17) and Her (8) presented excellent discussions of silica gel and vitreous silica properties. Depending on the preparation method, degree of internal ordering, and specific surface area, the solu bilities of vitreous silica and silica gel vary widely. However, both sub stances share similar ranges which are at least one order of magnitude greater than quartz i n the 0 ° - 2 5 ° C range of temperatures considered here. F o r this reason alone the two were lumped together i n the present discussion. Equilibrium values for the two i n seawater, p H 7.5-8.3, are i n the range 1500-2000 μΜ at 25° ± 1°C. The equilibrium solubility for low or alpha cristobalite i n distilled water at 2 5 ° C (by extrapolation from higher temperatures) is ca. 250 μΜ (18). In a later paper (19) these authors showed that this material was saturated sevenfold in distilled water at room temperatures and that the above extrapolated value was not attained during the experiment (4.5 years). Although we believe that the extrapolated value is valid, it Table I. Quartz

Values of Thermodynamic Properties 6

Low Cristobalite"

6°C 25°C S°C 25°C AG° 5.47 ± . 1 7 5.43 ± . 1 7 4.58 ± .2 4.58 ± .2 AH° 6.0 ± . 1 5 6.0 ± . 1 5 4.58 ± .3 4.58 ± .3 AS 1.91 ± .02 1.91 ± .02 ~0 ~0 The signs of all values are with respect to the reaction: (solid) —» silica monomer. From measurements of Morey et al. (19) From measurements of Fournier and Rowe (18) 0

β 6 c

Gibb; Analytical Methods in Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

18.

HURD AND THEYER

213

Biogenic Silica

is clear that it is difficult to attain equilibrium i n distilled water at room temperature. In his most recent paper, Fournier (20) give* high temperature ( 1 6 5 ° - 2 5 0 ° C ) solubilities for a high or beta cristobalite sample, which give lower temperature values (by extrapola tion) of ca. 1350 Μ at 2 5 ° C and 830 μΜ at 5 ° C . Stôber (15) d i d several experiments i n 0.154N sodium chloride solutions at 25 °C and ob tained values intermediate to the above-mentioned low and high cristobalite data. However, his sample was not well defined mineralogically, and his results are therefore questionable. N o data were found for seawater solubilities. Equilibrium solubilities for quartz i n distilled water at 25°C (also obtained by extrapolation from higher temperatures) are i n the range 100-200 μΜ (14,19, 21, 22); at 5 ° C by the same process, ca. 80-120 μΜ. The seawater solubility value of quartz at a slightly lower temperature (73 ± 5 μΜ at 2 0 ° C ) , obtained recently by Mackenzie and Gees (23), suggests that the data of Morey et al (19) are the most reliable. The latter s estimates are used in this paper. That each of the above forms (possibly excepting cristobalite i n distilled water) has a reasonably well defined and reproducible equi librium value suggests that the following familiar equations may be used to describe the net energy changes involved on reaching equilibrium (24):


μ

Change i n free energy (kcal/mole)

AG° = — RTln(K )

Change i n enthalpy (kcal/mole)

AH° =

(1)

eci

R

f,^*$

a{i/1

(2)

ab ; B

Change i n entropy (cal/deg K/mole) AS τ = (AH° - AG°)/(T ) &ha

(3)

Thermodynamic values at 5° and 25 °C for each of the three forms of silica are given in Table I. The large differences in these values suggest that the changes i n these properties as a function of the stability of the crystal structure should help to identify form changes i n the sediment. The most obvious change is that of solubility. Simply by cleaning the sample to remove clay minerals and absorbed cations and mixing the sample with seawater and allowing sufficient time for equilibration, as a Function of Temperature and Form High Cristobalite*

8

Biogenic Silica*

Vitreous Silica, Silica Gel'

5°C 25°C 3°C ~25°C 5°C 25°C 3.92 ± .2 3.92 ± .2 3.83 db .03 3.81 ± .03 3.70 ± .15 3.73 ± .15 3.64 ± .15 3.64 db .15 3.97 ± .58 3.97 ± .58 3.30 ± .03 3.30 ± .03 -0.96 ± .35 -0.96 ± .35 ~0 ~0 -1.44 ± .45 -1.44 db .45 From measurements of Fournier (20) From data in Appendix (this report) 1.0-6.4 X 10 years before present * From review of Alexander, Krauskopf, and Siever by Wollast (36) d

β

e


214



gross changes may be recognized easily in the surface properties of the biogenic assemblage per se. Changes i n heat of dissolution are generally within 10% of the absolute values of the free energy changes, but alter at a slightly different rate producing an interesting entropy effect. There is a net gain i n entropy when quartz dissolves and a net loss i n entropy when either vitreous silica or silica gel dissolves. That is, not only are greater heats of dissolution required to remove a silica molecule from increasingly more stable crystal structures, but the degree of disorder of the hydrated monomer relative to the molecule in the more stable crystal structure increases as well. The biogenic silica values were obtained from Table III for the time period Recent to 6.4 millions of years before Present ( m y b p ) . They show a net free energy change between that of cristobalite and the silica gel-vitreous silica combination as well as a similar intermediate status of enthalpy and entropy values. W e suggest that careful characterization of the solubility of a biogenic silica sample at several temperatures may yield useful information regarding its transformation to a more stable substance. Caution must be used i n interpreting these changes i n solu bility per se since: 1. A n y given assemblage contains on the order of three to five dozen different species of radiolarians, diatoms, and sponge spicules. Prelimi nary investigations based on the refractive index of each species (to be elaborated upon i n a subsequent report) suggests that almost every species is mineralogically slightly different from every other i n a given assemblage. 2. The range of specific surface areas from Recent radiolarians to sponge spicules may vary by nearly three orders of magnitude. Since Alexander (25) has shown that for a series of silica gel sols, solubility varies as a function of both specific surface area and internal structure, only general trends i n solubility can be discussed, and those, conserva tively. 3. Assuming that this process occurs by dissolution of the more soluble phase and precipitation of the less soluble ones, a relatively thin veneer of less soluble material may well coat those species which are mineralogically more stable, further tending to preserve them at the expense of the less stable ones. W h i l e the bulk of the assemblage may be thus coated, a smaller percentage may slowly still yield high solubili ties given long time intervals. This is further discussed i n the section on dissolution rates. Figures 1 and 2 show the change i n solubility of the acid-cleaned radiolarian and sponge spicule assemblages at 26° ± 1°C and 3° db 1°C as a function of geologic age. Also shown are the estimated values of the high and low cristobalite and low quartz solubilities at these two temperatures. The open and crossed circles represent the initial leveling off of the concentration of S i ( O H ) vs. time curves, and the dots are the 4



18.

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Biogenic Silica

215

values reached after three to six months of constant agitation. Although there is some scatter i n the data, there are clear trends of decreasing solubility with increasing sample age. A t least two types of behavior are apparent: a gradual decrease i n solubility w i t h increasing age, sug gesting at least by 60 rb 10 mybp cristobalite solubilities w i l l be reached and one i n which the process appears to be accelerated by a factor of three to four, and quartz solubilities are approached after only 15-20 mybp. Figure 3 shows the approximate age vs. number of occurrences of recrystallized cristobalite (porcelanite) and quartz (chert) found at selected sites of the Deep-Sea Drilling Project. The number within each box gives the site at which the mineral was found i n abundance. The age range of maximum occurrence of recrystallized silica forms (35-65 mybp) agrees quite well w i t h the solubility trends shown i n Figures 1 and 2. W o r k by Harder (26) suggests that quartz, i n the presence of vari ous metal hyroxides at p H 7 between 5° to 80 °C, spontaneously pre-

AGE OF SAMPLE, M.Y.B.P. Figure I .

Solubility of acid-cleaned biogenic silica in pH 8.3 seawater at 26° ± 1°C as a function of sample age. Samples from core S-68-24 are marked with ©; many of these samples showed low solubilities at much earlier ages than the other cores. # give dissolved silica concentrations for these same samples after an additional three to six months of constant agitation.


216


1400 3 ± rC pH 8.3 .SEA WATER.


Amorphous Silica

20 30 AGE OF SAMPLE, M.Y.B. P. Figure 2. Solubility of acid-cleaned biogenic silica in pH 8.3 seawater at 3° ± 1°C as a function of the age of the sample (symbols same as in Figure 1) to ο _

in Ο ^ U

Ζ

LU

υ u ο

72 34

168

169

70 221 33

166

153

63 219 165 135

217

62 212 146 144

216

66

136 146 61

71

Ο 4Η oc LU CQ

i 2 H

164 213

213 213 139 65

67 220 140

0 10 20 30 40 50 60 70 80 90 AGE OF DSDP SAMPLES HAVING CRISTOBALITE &/OR QUARTZ M.Y.B.P. Figure 3. Number of occurrences of recrystallized cristobalite or quartz from selected Deep-Sea Drilling Sites as a function of the age of the sample. Num bers within boxes refer to drilling sites. While the listing is not exhaustive, we do feel that it is representative.


18.

HURD AND T H E Y E R

217

Biogenic Silica

cipitated i n only a few weeks time from solutions w h i c h were much less than saturated with respect to silica gel. Since a l l deep-sea sediments which were squeezed at their in situ temperatures showed dissolved silica concentrations of not greater than 60% of amorphous silica satura tion at those temperatures, it is puzzling w h y quartz does not actually form more rapidly. Solution Rate Constants of the Various Forms of Silica


The first order congruent solution of amorphous silica is described by the equation dC/dt = fc (C 2

8at

- C i)S

(4)

BO

where k is the first order rate constant i n cm sec' , C t, the concentra tion of a solution saturated at a particular temperature and p H , and C i the solution being observed at time t i n moles cm" , and S is the surface area of the solid per unit volume of solution i n c m " ( J , 2, 22, 27, 28). There are a number of concepts to consider when using such a formula to describe the dissolution of biogenic silica. The importance of the surface area per unit volume term, S, cannot be overstressed. In the past virtually a l l investigators have lumped the k and S terms together, without knowing what the S term was. This then generates a countably infinite number of dissolution constants and dC/dt combinations, none of which can be compared with another (4, 5, JO, 28, 29, 30). T h e importance of the S term, then, is that the k value for the same substance under the same conditions of temperature, p H , and ionic strength is the same irrespective of the amount suspended i n solu tion. Only by knowing the S term can the surface properties of spines or shells from different organisms be compared since this allows calcu lation of k for a given set of conditions. 1

2

sa

s o

3

1

2

2

2

Figure 4 shows the change i n specific surface area of the acidcleaned assemblages with increasing age and reinforces the importance of determining the specific surface area of the solids involved. E v e n if no change i n crystal form occurred during a 40 million year period, a com parison of the dissolution rate of equal weights of sample (assuming incorrectly that their surface areas were the same) would show a differ ence i n initial solution rates of ca. a factor of 100. It is suggested that the observed two-orders-of-magnitude decrease results from both d i a genetic and morphological changes. Although w e are primarily concerned with these numbers insofar as they allow us to calculate S for a given experiment, they do allow us to quantitatively describe earlier micropaleontological observations relating to test structure such as "fragile" and "robust." Figures 5-10 show the degree of variation i n geometrical


218


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Changes in the Physical and Chemical Properties of Biogenic Silica

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