Changes which occurred during weathering of the Iowa Falls Member of the Mississippian Hampton Formation at Iowa Falls, Iowa were investigated quantitatively. The physical properties measured were: a) compressive strength, b) pore sized distribution, c) porosity and d) permeability. Chemically determined calcite and dolomite contents are related to the physical changes by utilization of an extensive sample sequence. The following changes are shown to be associated with the weathering process: 1) pore space and the degree of its interconnectedness increases with weathering, 2) calcite and dolomite are leached out during weathering, and 3) the rate of removal of calcite is twice that of dolomite

Elwell, J. H.

Lemish, J.

Sendlein, L. V. A.

Simon, D. E.

English

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 76 | Annual Issue Article 431969Measurement of Physical and Chemical ChangesInduced During Weathering of A Carbonate RockUnitD. E. SimonIowa State UniversityJ. H. ElwellIowa State UniversityL. V. A. SendleinIowa State UniversityJ. LemishIowa State UniversityCopyright © Copyright 1969 by the Iowa Academy of Science, Inc.Follow this and additional works at: http://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academyof Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu.Recommended CitationSimon, D. E.; Elwell, J. H.; Sendlein, L. V. A.; and Lemish, J. (1969) "Measurement of Physical and Chemical Changes Induced DuringWeathering of A Carbonate Rock Unit," Proceedings of the Iowa Academy of Science: Vol. 76: No. 1 , Article 43.Available at: http://scholarworks.uni.edu/pias/vol76/iss1/43Measurement of Physical and Chemical Changes Induced During Weathering of A Carbonate Rock Unit D. E. SrMoN1, J. H. ELWELL 2, L. v. A. SENDLEIN3 , AND J. LEMISH4 Abstract. Changes which occurred during weatherjng of the Iowa Falls Member of the Mississippian Hampton Formation at Iowa Falls, Iowa were investigated quantitatively. The physical properties measured were: a) cor:i-pressve strength, b) pore sized distribution, c) porosity and d) permeabil-ity. Chemically determined calcite and dolornte contents are related to the physical changes by utilization of an extensive sample sequence. The follow-ing changes are shown to be associated with the weathering process: 1) pore space and the degree of its interconnectedness increases with weathering, 2) ·calcite and dolomite are leached out during weathering, and 3) the rate of removal of calcite is twice that of dolomite. The materials of a weathered rock sequence are commonly re-lated through differences in their quantitative descriptions as fresh and altered rocks. However, the chemical changes actually occur-ring during the weathering process are currently only qualitatively understood because of difficulties encountered in measuring true gains and losses in a weathered sequence. The purpose of this paper is to demonstrate that weathering and sirnilar alteration processes which result in chemical and physical changes can be studied quantitatively. This can be accomplished by calculating gains and losses on the basis of a parent-daughter relationship between samples. The ordering of the rock sequence by use of an independ-ent parameter permits plotting these data so calculated gains and losses are presented graphically as functions of the degree of weath-ering. This provides a better picture of the tme nature of the proc-esses at work. This paper demonstrates how use of a variety of data and application of a constant container volume concept, (which stresses the self consi~tent parent-daughter relationship of a weath-ering sequence), makes it possible to obtain the type of data nec-essary to quantitatively understand the weathering processes. The system chosen for this study represents a gradational se-quence from fresh to severely weathered carbonate rock from .the 1 Graduate Teaching Assistant, Department of Earth Science, Iowa State University, Ames, Iowa 50010. 2 Instructor, Department of Earth Science, Iowa State University, Ames, Iowa 50010. 3 Associate Professor, Department of Earth Science, Iowa State Univer-sity, Ames, Iowa 50010. 4 Professor, Department of Earth Science, Iowa State University. Ames, Iowa 50010. · · 320 1Simon et al.: Measurement of Physical and Chemical Changes Induced During WeathPublished by UNI ScholarWorks, 19691969] CHANGES DURING WEATHERING 321 dolomitic Iowa Falls member of the Mississippian Hampton Forma-tion in the Kinderhook Series exposed at Iowa Falls, Iowa. The samples were labelled as follows: 1) Sample IF-24, fresh rock, 2) 3) Sample IF-24W, intermediately weathered rock, and Sample IF-W, severely weathered rock PHYSICAL PROPERTY CHANGES One of the physical properties measured was the compressive strength and the results are shown in Figure 1 as stress--strain curves. The results indicate that Sample IF -24 is strongest, Sample IF-24W intermediate in strength, and Sample IF-W is weakest. As suspected strength decreases as weathering progresses. 0 0 -- ff-24 ----" W-~4W _.,....,.. lf·W 2 ~ °"4''69 10 12 S'.TRAIN Al./Lt~I02) Fig. 1. Stress-strain relationships for samples. Figure 2 shows the pore size distributions for the samples as determined by mercury injection techniques (Hiltrop and Le~ish 1959). The distributions are normalized and show that weathenng 2Proceedings of the Iowa Academy of Science, Vol. 76 [1969], No. 1, Art. 43http://scholarworks.uni.edu/pias/vol76/iss1/43322 IOWA ACADEMY OF SCIENCE [Vol. 76 --- •-24n -- - 11"-W OJ 0.5 1.0 PORE' t)~~~ ~rf~l.)S. .(M«;~lj Fig. 2. Pore size distributions. creates larger pore entry radii at the expense of smaller ones as demonstrated by the shift of the distribution maximum toward larger pores coincident with the increased degree of weathering. Table 1. Summary of the physical properties for the sample sequence Property IF-24 IF-24W IF-W Degree of Wathering Fresh Intermediate Severe Porosity-Hg ( % ) 2.37 14.96 25.30 Porosity-H20 ( % ) 4.76 20.08 32.15 Bulk Density (g/cc) 2.637 2.266 1.867 Rock Density ( g/ cc) 2.698 2.657 2.545 Permeability ( darcy) 0.011 0.096 0.300 Yield Strength (psi xlO') 6.40 3.75 0.75 Fracture Strength (psi x 101 ) 7.75 5.10 1.43 Physical properties measured are listed in Table 1. From the values obtained for each property the following consistent trends for the unwea.thered to weathered sequence are: 3Simon et al.: Measurement of Physical and Chemical Changes Induced During WeathPublished by UNI ScholarWorks, 1969196,9] 1) 2) CHANGES DURING WEATHERING density is decreasing, strength is decreasing, 3) porosity is increasing, 4) permeability is increasing, and as was shown earlier, 323 5) the pore entry radius maxima indicate larger pores and broader pore size distributions in the weathered rock. These observations establish the quantitative aspects of physical changes and focus attention on the problem concerning the direc-tion of net chemical changes accomplished by the weathering process. CHEMICAL PROPERTY CHANGES On an atomic scale the results of the weathering process are expressed as gains and losses of chemical elements between fresh and weathered rocks of a sequence. The chemical composition of these rock materials can be determined and may be reported as weight-percent data. Differences in these weight-percent data can be used to recognize the presence of a change and to infer differ-ences between samples of a weathering sequence. This traditional approach is contrasted with the application of the proposed unit container volume concept which stresses the self consistent parent-daughter relationships of the weathering sequence. Weight-Percent Approach The samples were analyzed by procedures outlined by Bisque (1961), and the results are presented in Table 2. In order to Table 2. Table of chemical analysis data for sample sequence Weight Percent Oxide IF-24 IF-24W IF-W Cao 32.06 31.40 31.48 MgO 18.26 18.67 18.96 co, 45.09 45.01 45.39 Insoluble Residue 0.09 0.18 0.23 Fe,O, 0.23 0.52 1.13 Calcite1 11.89 9.71 9.11 Dolomite' 83.51 85.37 86.70 'Calcite= [CaO ~ MgO (1.3908)] [l.7847] 'Dolomite = (MgO) (4.5734) demonstrate graphically the chemical changes associated with weathering, the porosity of the samples is used as an independent parameter in Figures 3, 4, and 5. Porosity was chosen because it shows an approximate linear trend as weathering progresses. Figure 4Proceedings of the Iowa Academy of Science, Vol. 76 [1969], No. 1, Art. 43http://scholarworks.uni.edu/pias/vol76/iss1/43324 IOWA ACADEMY OF SCIENCE [Vol. 76 3, a plot of chemical data for the major mineral phases as re-ported for each sample, shows that dolomite content increases and calcite decreases with weathering expressed as increasing porosity. The slope ( m ) of a line connecting Samples IF -24 and IF -W is given as an aid in comparing the magnitude of change. In the re-porting of weight percent data the percentages of all components present in each sample must sum to one hundred percent. This is shown in Figure 3 by the mass being one hundred for all samples since on this basis the percentage shown is numerically equal to its mass in grams. Because each sample must sum to 100, a change in any one component will influence the value reported for all other components. As a result of the above mathematical restriction there exists several ways to account for variations in the reported weight percentages for a sample sequence. In this sample sequence several equally valid interpretations for mineral changes occurring during weathering would yield the same weight percent results. Some of these interpretations are: 1) the dolomite content is increasing, 2) the calcite content is decreasing, 3) both of the above occur simultaneously, 8 ~ 16 0 24 28 9 (%1 IF'-24• '-"" fC W INTEFIMEOIAT!: SEVERE Fig. 3. Comparison by weight-percent approach. 5Simon et al.: Measurement of Physical and Chemical Changes Induced During WeathPublished by UNI ScholarWorks, 19691969] CHANGES DURING WEATHERING 325 4) both calcite and dolomite are being added, but at different rates, and 5) both calcite and dolomite are being removed, but at differ-ent rates. Herein lies the inherent dilemma associated with the use of weight percent data for comparative purposes. Which of the above inter-pretations best accounts for physical changes? U nit-Container-Volume Approach Another approach, which gives an accurate picture, is to con-sider th~ weathering process in terms of a sample container of unit volume. This idealized volume will be constant during weather-ing. Materials leave or enter this container depending on the proc-ess operative. One can follow the changes on the basis of a parent-daughter relationship between materials of a sample sequence by comparing the mass present within the unit container volume of each material. Consider the fresh rock as a parent, the interme-diate weathered rock as a first daughter, and the severely weath-ered rock as a second daughter, etc. The mass within a unit con-Z50 0.'0 0 :I - IF·l~ FRE:Siol ' . -~ 8 ~ ~ ~ ~ ~ POROSITY- HQ ·£%t Fig. 4. Comparison by unit-container-volume approach. 6Proceedings of the Iowa Academy of Science, Vol. 76 [1969], No. 1, Art. 43http://scholarworks.uni.edu/pias/vol76/iss1/43326 IOWA ACADEMY OF 'SCIENCE [Vol. 76 tainer volume of any material may be calculated by dividing the total mass of a core sample by its volume. Mass differences. should show the kinds and directions of chemical changes during weather-ing (Elwell, Simon, Lemish, 1968). In Figure 4, the observed chemical changes for the two major mineral phases of the dolomitic rock are determined for a parent-daughter relationship. IF-24 is the parent, IF-24W is daughter I, IF-W is daughter II. An important aspect is the effect of mass action. The Law of Mass Action essentially states the greater the quantity of a material available for reaction, the larger the amount of material participating in the reaction. Because the rock is mostly dolomite, net gains and losses of the rock most closely reflect changes in this mineral. This is seen in Figure 4 by comparison of the slopes for dolomite and calcite with the total mass change. Note that on a rock basis, calcite and dolomite are both pro-gressively removed from the rock during weathering, coincident with a decrease of total mass in the unit container volume. From this observation we can immediately reject all the interpretations deduced from weight percent comparisons but the 5th, which stated that calcite and dolomite are being removed but at different relative rates. Chemical Reactivity of the Minerals Another advantage of the unit container volume approach is that relative rates of removal for each mineral in the container independent of mass action can be quantitatively shown. Because we are dealing with a net material balance between parent and daughter materials, relative changes in the rates of removal of a mineral may be obtained experimentally without knowledge of the degree of attainment of solution equilibria. In this case the effect of mass action is removed by using sets of samples of different sizes such that each parent material contained 100 moles of a given mineral to be studied. Then the ratio of moles of this mineral re-maining in each daughter sample to the 100 moles in the parent results in plots as shown in Figure 5. The amount of calcite re-moved relative to the amount originally present is 40 percent. How-ever, the amount of dolomite removed is only 20 percent. The rela-tive rate of leaching for the calcite is two moles for each mole of dolomite removed which shows that calcite is preferentially ex-tracted. Is this observation consistent with outcrop and petrographic observations? PETROGRAPHIC OBSERVATIONS Petrographic study of the sample sequence qualitatively in-dicates that interstitial calcite is being progressively removed from 7Simon et al.: Measurement of Physical and Chemical Changes Induced During WeathPublished by UNI ScholarWorks, 19691969] CHANGES DURING WEATHERING 327 "'T -.--- I 20 2~ 28 ... . , ... ,~ 0 <I 9 IZ I 20 24 28 POROSI T'r - H ~ I'll.I Fig. 5. Molar ratios for calcite and dolomite of parent-daughter samples. the dolomite matrix. Visual observation at the sampling site for IF -W showed that the final weathering product is an unconsol-idated dolomite sand. Mason (1961) also found this to be the case at other outcrops of this Iowa Falls member. Thus this in-dependent qualitative information is consistent with the calculated quantitative changes. DISCUSSION The physical properties measured showed changes occurring during weathering of the dolomitic rock. These changes included development of higher porosity, higher permeability, and larger pores along with a decrease in the density and strength. Petro-graphic evidence qualitatively indicated that interstitial calcite was progressively removed resulting in a dolomite sand. The comparison of chemical weight percent data between samples did not indicate any definitive process. Through the use of the proposed unit con-tainer volume concept it was quantitatively shown that both calcite and dolomite were removed from the rock during weathering. The quantity of dolomite removed from the rock is greater than the 8Proceedings of the Iowa Academy of Science, Vol. 76 [1969], No. 1, Art. 43http://scholarworks.uni.edu/pias/vol76/iss1/43328 IOWA ACADEMY OF SCIENCE [Vol. 76 quantity of calcite. However, the relative rate of removal for the individual mineral phases, neglecting their abundance showed two moles of calcite removed for each mole of dolomite. This indicates that the mineral calcite is preferentially removed, even though it is less abundant in the rock. Once it is known that both dolomite and calcite minerals are being removed from the rock the problem of naming the net op-erable process is made easier. If dolomitization and dedolomitiza-tion are restricted to changes in carbonate structures both of these processes may be rejected. Dolomitization is rej•ected because of the net loss of magnesium from the rock. Dedolomitization is also re-jected because of the preferential loss of the calcite mineral. In this case the increasing relative abundance of dolomite in respect to calcite is best explained as residual enrichment, resulting from the selective removal of calcite. CONCLUSIONS Application of new methods and the combination of several types of measuring has permitted the determination of quantita-tive changes during weathering. Consideration of all data demon-strates some important conclusions regarding weathering which may be looked at from both a rock and a mineral level. 1) On the rock le,·el, bulk changes show that most of the material removed is dolomite. 2) However, on a mineral level calcite is removed faster than dolomite, and results in a sanded dolomite. 3) Thus the dolomite is being residually concentrated even though it is the bulk of the material removed. 4) The relationship of these chemical and physical changes indicate the manner and rate at which material is re-moved from the rock during weathering. The removal of material explains the obserYed decreasing density and strength and the increasing porosity, permeability, and pore entry radii. 5) Although standard chemical and petrographic methods qualitatively show that weathering increases the relative abundance of dolomite, it is only the proposed unit con-tainer volume method that permits quantitative detection of the process responsible for net chemical and physical changes. 6) The unit container volume method provides a basis for a material balance so that the analytical results may be re-lated between samples as gains and loses of materials. This problem is an old one. Krauskopf and others have rec-9Simon et al.: Measurement of Physical and Chemical Changes Induced During WeathPublished by UNI ScholarWorks, 19691969] CHANGES DURING WEATHERING 329 ognized all that is necessary to solve the dilemma of weight percent data is "to have independent evidence as to how the mass or volume of the rock has changed during weathering" (Krauskopf, 1967). Our method now provides this evidence. REFERENCES CITED BISQUE, R. E., 1961 "Analysis of Carbonate Rocks for Calcium, Magne-sium. Iron, and Aluminum with EDTA", four. Sed. Pet. Vol. 31, p. 113-122. ELWELL, J. H., D. E. SIMON, & J. LEMISH, 1968 "Approach to Evaluation of Geochemical Change: Abstract in North Central Geological Society of America! Program, Iowa Cit]', Iowa, p .. 22. HILTROP, C. L. & J. LEMISH, 1959, "The relationship of pore-size distribu-tion and other rock properties to the serviceability of some carbonate rock", H.R.B. Bull. 239, p. 1-23. KRAUSKOPF, KoNRAD B., 1967, Introduction to Geochemistry. McGraw-Hill, Book Co., New York. MASON, J. R., 1961, "Petrology of the Hampton Formation at fowa Falls, Iowa." Unpublished M.E. Thesis, Library, Iowa State University of Science and Technology, Ames, Iowa. 10Proceedings of the Iowa Academy of Science, Vol. 76 [1969], No. 1, Art. 43http://scholarworks.uni.edu/pias/vol76/iss1/43

Measurement of Physical and Chemical Changes Induced During Weathering of A Carbonate Rock Unit

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Measurement of Physical and Chemical Changes Induced During Weathering of A Carbonate Rock Unit

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