JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B6, PAGES 7321-7331, JUNE 10, 1989

Are the Iron Carbonate Minerals, Ankerite and Ferroan Dolomite, Like Siderite,

Important in Paleomagnetism?

BROOKS B. ELLWOOD, 1 BURKE BURKART, 1 KRISHNAN RAIESHWAR, 2 ROBERT L. DARWIN, 1 RICHARD A. NEELEy1, ALDEN B. MCCALL,1 GARY J. LONG,3 MARGARET L. B UHL,3

and CHARLES W. HICKCOX

Magnetic, M6ssbauer effect, and thermal properties have been evaluated for specimens of the mineral ankerite and are contrasted with slimilar measur,?,ents m•ned for Lhe mi. Lnera! siderite to dete__rm_ ine if ankerite (and ferroan dolomite), like siderite, may be important in paleomagnetism as a producer of secondary, spurious remanent moments as the result of oxidation in air. Our data indicate that ankerite and siderite both have comparable thermal stabilities and that ankerite does break down to form iron oxides at temperatures as low as 250øC during thermal demagnetization. We conclude from our data that any iron carbonate, siderite, ankerite, or ferroan dolomite, may readily oxidize in air to maghemite or hematite thus producing a spurious remanent moment (RM) that can dominate the magnetic properties of the specimen. Thermal demagnetization of ankerite or siderite at lower temperatures (below 250øC) does not appear to produce such a spurious RM. As might be expected, the quantity of secondary iron oxides produced by oxidation is limited by the iron content in the carbonate crystal lattice. Oxidized siderite produces significantly greater amounts of iron oxides than equivalent percentages of ankerite, which in tum produces more iron oxides than equal amounts of ferroan dolomite in a given sample.

INTRODUCTION

It has been established that the iron carbonate mineral

siderite, is chemically unstable, and upon exposure to air, oxidizes to maghemite and magnetite, thus changing the magnetic properties of the host rock [Ellwood et al., 1986, also An experiment to evaluate the magnetic effects resulting from oxidation of siderite in air, submitted to Journal of Geophysical Research, hereinafter referred to as B.B. Ellwood et al., submitted manuscript, 1988]. These changes can be dramatic, including large variations in sus- ceptibilities and remanent moment (RM), large changes in magnetic fabric, and even self-reversals of RM directions. The implications for paleomagnetic studies have a good side but also a bad side. If siderite is present, indicating that oxidation has not affected the rock, and if samples are handled very carefully, then it is possible that siderite can be an indicator of samples that should produce reliable paleomagnetic results. Demagnetization techniques can then be designed to reduce oxidation or eliminate its effects [Ellwood et al., 1988]. On the other hand, oxidation of siderite before or during paleomagnetic sampling and measurement can produce secondary (spurious) magnetic phases which may dominate the RM.

It is important to determine whether unstable phases such as siderite are present, because of the potential magnetic changes produced during laboratory exper- imentation. The anisotropy of magnetic susceptibility (AMS) signature of the rock can aid in this identification.

1 Department of Geology, University of Texas at Arlington. 2 Department of Chemistry, University of Texas at Arlington. 3 Department of Chemistry, University of Missouri-Rolla. 4 Department of Geology, Emory University, Atlanta, Georgia.

Copyright 1989 by the American Geophysical Union.

Paper number 89 JB00044. 0148-0227/89/89 JB -00044505.00

For example, it was the AMS data that provided the key in identifying siderite as the mineral responsible for the unusual magnetic properties of the Austin Chalk in Texas [Ellwood et al., 1986]. Their observations were based on similar unusual characteristics associated with oil sand

samples from the Athabasca Oil Sands, Alberta, Canada, containing siderite [Ellwood and Pemberton, 1984]. Oxidation studies (the result of heating in air) of siderite ore samples from Austria and Nova Scotia have shown a number of striking features, including a complete in- terchange between the maximum and minimum AMS directions as siderite is converted to a secondary iron oxide [Ellwood et al., 1986]. This interchange occurs at temperatures above 300øC.

Iron carbonates. Siderite (FeCO3) is an isomorph of calcite (CaCO3). Two other carbonates, dolomite, (Ca,Mg)(CO3)2, and ankerite, Ca(Mg,Fe)(CO3)2, both are common and isomorphous with each other. Ankerite forms a solid solutions series with dolomite, Fe substituting for Mg. The term "ferroan dolomite" refers to that part of the series where Fe 2+ (or Fe 2+ and Mn 2+) fills fewer than 20% of the cation positions of the M g-Fe layer. Ankerite is thereby arbitrarily defined as having an Fe/Mg ratio of greater than 1/4 [Deer et al., 1966].

Ankerite is a difficult mineral to identify in sedimentary rocks because it has optical properties and crystalline structure similar to dolomite. Special staining techniques, X ray diffraction (XRD), chemical analysis, microprobe analysis, or refractive index studies are required for positive identification. Fortunately, because the M6ssbauer effect hyperfine parameters are very sensitive to the local site symmetry, they may be used to identify and separate the contributions of ankerite and siderite in a mineral if the

iron concentrations are a few percent or higher [Price, 1978; DeGrave and Vochten, 1985]. When ankerite concentrations are very low, the problem is more acute.

Dolomite, and therefore presumably ankerite, is currently forming in ocean sediments in many areas and is

7321

7322 Ellwood et al.: Ankerite and Ferroan Dolomite

controlled by the depositional environment [Baker and Burns, 1985]. Usually, this environment is organic rich with sulfur in the reduced sulfide state. Production rate of dolomite is limited by magnesium and bicarbonate concentrations within the sediment; or in the case of ankerite, we infer that its production will be limited by the iron/magnesium ratio and bicarbonate concentration.

A number of other natural occurrences of the mineral have been reported, including work by Boles [1978], where he identified ankerite as a cement replacing calcite in sandstones of the Wilcox Formation in southwest Texas. Boles has suggested that this replacement occurs at tem- peratures of 125oC to as high as 210oC. Ankerite has also been identified in oil shales of the Mahogany Zone of the Green River Formation, Wyoming, where "dolomitic minerals" make up 35-40% of the mineral constituents. Because of extremely fine grain sizes (-•5 gm), ankerite requires XRD identification [Smith and Robb, 1966]. We have identified ankerite as a cement in the Strawn Sand of

Archer County, Texas, and as the material filling some dinosaur footprints in Dinosaur State Park near Glen Rose, Texas, where the ankerite appears to have formed by penecontemporaneous precipitation directly from water at the sediment-water interface. In the Strawn Sand, the ankerite has been incorrectly identified as dolomite by two commercial laboratories using XRD data (unpublished data), because they incorrectly interpreted the shift in the primary dolomite peak to be the result of instrumental error, instead of resulting from the presence of ankerite. In the Glen Rose ankerite occurrence, crystal morphology has generally been interpreted by geologists to represent the presence of dolomite, because previous identifications have not been supported by XRD data. It is very possible that many such ankerite occurrences have been incorrectly identified as dolomite; therefore ankerite may be much more common than previously thought.

The Problem. Paleomagnetic studies of siderite have indicated that it readily oxidizes in air to form secondary, magnetic iron oxides [Ellwood et al., 1986]. Furthermore, it has also been suggested that ferroan dolomite, during chemical breakdown or dedolomitization, may produce other paleomagnetically important but secondary iron- bearing minerals [e.g., Elmore et al., 1985]. Ferroan dolomite may contain significantly less iron than ankerite, so while it is important to identify the stability of both of these iron carbonates to understand their contribution to the magnetic properties of rocks, a study of ankerite is a logical first step. To evaluate the significance of ankerite in magnetic studies, we have collected and examined coarse crystalline ankerite samples from the Erzberg Mine in Austria, coarse crystalline ankerite cement from the Strawn Sand in Texas, and fine crystalline ankerite from Glen Rose, Texas. The data reported here contrast the chemical, thermal, and magnetic properties of these ankerite samples with siderite. The results are compared to chemical and magnetic data for calcite samples from Carlsbad Cavern, New Mexico, and nonferroan dolomite samples from Utah.

METHODS

Paleomagnetic cores (-2.5-cm in diameter) of siderite and ankerite were drilled from unoriented hand samples collected from the Erzberg Mine in southem Austria. Also

cored were calcite samples from a standing speleothem in Carlsbad Cavern, New Mexico; a series of dolomite samples from the Sevy Dolomite in west central Utah; samples containing ankerite cement from a core of the Strawn Sand, Archer County, Texas; and ankerite-rich sediment samples; the material filling some dinosaur footprints at Dinosaur Valley State Park near Glen Rose, Texas.

Magnetic studies. Paleomagnetic cores were sliced to -•2.2-cm length, and the natural RM was measured using a Molspin spinner magnetometer. The AMS was measured in low magnetic fields (<8 mT) by using two automated, low-field, torsion-fiber magnetometers [King and Rees, 1962; Stone, 1963]. AMS is commonly expressed as an ellipsoid with principal Z1 (maximum),Z2 (intermediate), and Z3 (minimum) axes, which in general terms reflect the physical or crystallographic orientation of magnetic elements in a single sample [Nye, 1969]. In many rock samples, the AMS ellipsoid has been shown to reflect the petrofabric orientation [e.g., Khan, 1962; Taira and Lienert, 1979], which results from any process that physically orients elements within the sample, such as crystal growth, flow, diagenesis, or stress. Susceptibility is given as

Z = (Z 1 + Z2 + Z3)/3 (1)

The AMS ellipsoid shape expresses either the preferred alignment of included magnetic grains or, as was demonstrated for siderite samples [Ellwood et al., 1986] the overall crystallographic mineral orientation in the whole sample. This crystallographic orientation may or may not correspond to the mineral elongation or orientation in the sample. We use the parameter V, developed by Graham [1966], as an indicator of shape, where

sin2V = (Z 2 - Z3)/(Zl - Z3) (2)

and V < 45 ø for prolate shapes and >45 ø for oblate shapes. Heat treatment. After initial chemical and magnetic

measurements, a series of thermal experiments were performed. These consisted of heating samples at 100øC increments to 600øC in a Schonstedt thermal specimen demagnetizer (TSD-1) and remeasuring the RM and AMS after each treatment.

Analytical studies. Analytical studies were conducted before and after heat treatment on small amounts of crushed

samples. XRD analysis was performed on a Philips diffractometer utilizing CuK a radiation. Total iron was determined using a 1985 version of the Varian 975 Flame/Furnace Atomic Absorption (AA) Spec- trophotometer. X ray florescence (XRF) analyses were used to identify total sulfur and iron concentrations in siderite and ankerite samples.

M6ssbauer effect spectra were obtained using a Harwell constant acceleration spectrometer which was calibrated with room temperature natural abundance a-iron foil and utilized a rhodium matrix cobalt 57 source. The spectra, as a function of temperature, were measured in a Janis Supervaritemp cryostat with a Lake Shore Cryotronics temperature controller. These spectra were fit to symmetric

Ellwood et al.: Ankerite and Ferroan Dolomite 7323

quadrupole doublets with Lorentian line shapes by using standard least squares computer minimization techniques. The resulting hyperfine spectral parameters are accurate to approximately 0.02 mm/s.

Differential thermal analysis (DTA) and thermogravimetry (TGA) measurements were performed to compare and contrast the thermal stability of the carbonate minerals. A DuPont Model 9900 system fitted with the appropriate accessory modules was used for these measurements. Sample sizes were in the 10-20 mg range. Static air was used for all scans. The heating rate was 20øC/min for DTA and !5øC/rain for TGA ana!y•e•_ In comparing the data for siderite and ankerite discussed below, the 5øC difference in heating rates has been considered.

RESULTS

XRD, AA, and XRF Data. AA data indicate that the calcite and dolomite samples reported here contain no iron. Boles [1978] notes that in the solid solution between dolomite and ankerite, the d104 inter-atomic spacings vary linearly with the Fe/Mg ratio (Figure 1). Larger d spacings result from increased incorporation of the larger Fe 2+ (0.76/•) over Mg 2+ (0.65/•) ions. The Erzberg ankerite sample falls on the Boles [1978] curve, with a 2.909/• d spacing for the (104) diffraction maximum (Figure 1). The MOssbauer, XRD, XRF, and AA dam confirm the Erzberg sample contains very pure ankerite, therefore with -53 mol % Fe 2+ relative to the (Mg,Fe) concentration in the ankerite crystal lattice. The sample from the Strawn Sand (AM4366) contains 6.90 wt % total Fe, while the Glen Rose ankerite sample contains only 1.15 wt % total Fe. The XDR data for these samples are plotted in Figure 1, which has been redrawn from Boles [1978]. The dashed line in Figure 1 is constrained to pass through the dolomite sample (ASTM 11-78) and the Erzberg ankerite ore sample. A linear fit of the Erzberg, two ASTM samples and two of the Wilcox samples is essentially coincident with this dashed line. These data suggest that it may be possible to estimate the mole percent of Fe in the ankerite crystal lattice from the XRD d spacing. When the sample lies to the fight of the dashed line in Figure 1, we infer that excess iron is present in some other iron-bearing mineral such as chlorite or pyrite.

The presence of sulfur was measured to estimate the possible contamination of pyrite or other iron sulfides in the samples analyzed, since these sulfides might affect the magnetic data. The XRF data indicate that sulfur percentage is very low (<1%) in all samples studied. If it is assumed that all the sulfur in these samples is tied up in pyrite, then our results indicate that at most the ankerite ore sample could contain <0.07 wt % pyrite; siderite ore <0.13%; Strawn Sand (AM4366)<0.29%; and the Glen Rose sample <0.82% pyrite. Furthermore, the MOssbauer data indicate that the ore samples of ankerite and siderite used in this study do not contain other detectable Fe2+ minerals (Figure 2).

MOssbauer effect data. The M0ssbauer effect spectra were measured as a function of temperature to faciiitate direct comparison of the hyperfine spectral parameters of our samples of ankerite and siderite and for comparison

2.920

2.915

2.910

o •, 2.905

03 2.900

• 2.895

2.890

- /ASTM •' 11-78

0 10

I

/

/ I

I

'• WILCOX /

STRAWN AM 4366

/

/

/

GLEN ROSE •/ e/ERZBERG

/

/

e?WILCOX /

/

/ ASTM / ©12-88

ß

WILCOX

20 30 40 50 60 70 80

Mole % Feof Total Moles (Fe + Mg) Fig. 1. Variation in d lattice spacing for dolomite-ankerite series samples. Wilcox data points are from Boles [1978]; ASTM values from Berry [1974]; Erzberg ore sample from this paper (see text); Glen Rose and Strawn sample data points are based only on d spacing and intersect the dashed line at star points. From these data the mole percent Fe can then be estimated.

with the results reported by DeGrave and Vochten [ 1985]. The resulting MOssbauer hyperfine parameters are presented in Table 1 and the room temperature spectra are shown in Figure 2. Note, as discussed above, that these spectra are very clean, indicating essentially no Fe 2+ contamination by other minerals such as pyrite. The asymmetry observed in the areas of the two components of the quadrupole doublet is independent of temperature in both ankerite and siderite and results from sample texture effects, as demonstrated by DeGrave and Vochten [1985]. The temperature dependence of the hyperfine parameters and the MOssbauer absorption area [Long, 1984] are illustrated in Figure 3 and the linear fits to the data are given in Table 2. Also given in Table 2 is the effective mass, Meff, of the iron 57 nuclide, and the MOssbauer temperature, 0M, as calculated by using the expressions reported by Herber [1984].

Thermal analyses. Representative DTA curves and TGA profiles for siderite (Figure 4a) and for ankerite (Figure 4b_.) indicate that the endothermic features in DTA are in good accord with the corresponding processes seen in the TGA scan, given the differences in the spatial location of the thermal sensor in the two sets of experiments. The siderite DTA endotherm (Figure 4a_) at-520øC and the TGA features in the 500ø-600øC range are attributed to the

7324 Ellwood et al.: Ankerite and Ferroan Dolomite

RELATIVE ENERGY (JxlO •) -3 -2 -1 o 1 2 3

g

o

100.0

98.0

96.0

94.0

100.O

98.0

96.0

94.0

92.0

90.0

ANKERITE

290K

SIDERITE

290K

t I t I I I I I I I I I i I I I I I :

-4 -3 -2 -1 0 1 2 3 4 5

SOURCE VELOC!TY (HH/S)

Fig. 2. M6ssbauer effect spectra for ankedte and siderite samples obtained from Errl>erg, Austria.

breakdown of iron carbonate in the siderite matrix. DTA

data suggest a secondary mineral present in the siderite sample. This appears as a second (minor) endotherm at -780øC, and a correlated TGA weight loss at 650ø-750øC (Figure 4a_). The XRD data indicate that a small amount of ankerite may be present in the siderite ore sample, and 780øC is essentially identical to the magnesium carbonate breakdown temperature (-785øC) exhibited by the ankerite sample discussed below (Figure 4b). The slight increase in mass in the TGA curve in Figure 4a at temperatures >750øC is attributed to the oxidation of FeO formed earlier

during siderite breakdown. Figure 4b illustrates analogous data for an Erzberg

ankerite sample. The DTA and TGA data exhibit three distinct variations. The weak endothermic DTA feature

beginning at-470øC (Figure 4b_), accompanied by a large weight loss in the TGA profiles, we attribute to the breakdown of iron carbonate in the ankerite matrix. This

temperature is slightly lower than for siderite samples (Figure 4a_.) and not as well defined. At -700øC, weight loss, due to magnesium carbonate dissociation, culminates in a peak at-785øC in the DTA (Figure 4.b_). A third feature at -845øC in the DTA curve is attributed to the breakdown of calcium carbonate in the ankerite.

Comparisons of the relative weight losses due to the iron and magnesium carbonate components of the ankerite

TABLE 1. M6ssbauer Effect Hyperfine Spectral Parameters

T, •, AE_• F1, F 2, Area, OK mm/s mm/s mm/s mm/s (%e)*(mm/s)

Ankerite

290 1.245 1.462 0.33 0.34 7.25 275 1.255 1.489 0.34 0.34 7.36 250 1.271 1.534 0.33 0.34 7.56 225 1.286 1.578 0.34 0.34 7.76 200 1.304 1.627 0.34 0.36 8.10

175 1.319 1.672 0.35 0.35 8.29 150 1.332 1.722 0.34 0.35 8.42 125 1.347 1.769 0.34 0.36 8.58 100 1.358 1.821 0.36 0.36 8.65

85 1.363 1.854 0.35 0.37 8.77 78 1.366 1.867 0.33 0.35 ....

Siderite

290 1.234 1.802 0.30 0.31 10.67 275 1.244 1.819 0.32 0.30 10.90 250 1.260 1.844 0.31 0.32 11.14 225 1.276 1.870 0.31 0.32 11.44 200 1.291 1.894 0.32 0.33 11.73 175 1.308 1.924 0.32 0.33 12.02 150 1.322 1.954 0.32 0.33 12.14 125 1.334 1.982 0.33 0.34 12.50 100 1.344 2.008 0.32 0.35 12.72

85 1.349 2.020 0.33 0.35 12.86

-aRelative to room temperature natural abundance {z-iron foil; see text for a detailed explanation of parameters used.

Ellwood et al.: Ankerite and Ferroan Dolomite 7325

I I I I i

2.00 - • -

E 1.90 - e• IDERITE -

,=,=,

•. 1.80- • -

O

• 1.70 - -

o

1.6o - -

1.5o - -

• 1.40 - E E ,,• 1.35 - -

03 1.30 - -

E

O 1.25 - SIDERITE • _

2.30 - -

• SIDERITE

< _1 2.10- • '""-t.. -

2.00 - ANKERITE • _

0 50 100 150 200 250 300

Temperature, øK

Fig. 3. The temperature dependence of the isomer shift, quadrupole splitting, and In area for the ankerite and sideritc samples.

matrix in Figure 4b allows us to estimate that roughly half of the Mg 2+ sites are replaced by Fe 2+ in the sample. This estimate is similar to that we have determined from the

XRD, XRF, and AA data. We have also performed DTA analyses on the dolomite and calcite specimens reported in this study for comparison and to aid in our interpretation of the siderite and ankerite thermal data. Our DTA and TGA results for these minerals are consistent with those results

reported elsewhere [e.g., Smykatz-Kloss, 1974; Todor,

1976] but add nothing new to our interpretation of the data in Figure 4.

Magnetic data. Heating of ankerite samples demonstrates that similar magnetic changes are observed for both minerals with the exception that the AMS axes in the ankerite samples so far analyzed change at slightly higher temperatures than in the siderite samples. RM and sus- ceptibility values increase in both minerals above 250øC (Figure 5). For all siderite samples that we have analyzed

7326 Ellwood et al.: Ankerite and Ferroan Dolomite

TABLE 2. Temperature Dependence of the M6ssbauer Spectral Parameters

Parameter Ankerite Siderite

Isomer Shift

slope (mm/s)/K -5.88 x 10 -4 -5.73 x 10 -4 intercept, mm/s 1.418 1.404 correlation coefficient 0.998 0.997

Quadrupole Splitting

slope (mm/s)/K -1.90 x 10 -3 -1.08 x 10 -3 intercept, mm/s 2.009 2.114 correlation coefficient 1.000 1.000

In Area

slope (mm/s)/K -10.17 x 10 -4 -8.97 x 10 -4 intercept, mm/s 2.279 2.635 correlation coefficient 0.996 0.997

]•[eff Daltons a_ 70.8 72.6

0M K b 328.0 345.0

a_Obtained from Herber [1984], equation (16). b Obtained from Herber [1984], equation (18); see text for a more

deta•ed explanation of the parameters used.

this increase is much greater than for ankerite samples. (Note, similar data for other siderile samples were reported by Ellwood et al. [1986].) However, inversion of the Z1 and Z3 AMS principal axes begins at 300ø-350øC in siderite samples, but not until 400ø-450øC for ankerite samples, and is complete by 400 ø or 500øC, respectively (Figures 6a and 6!2). No equivalent inversion is observed for either calcite or dolomite samples with heating to 600øC (Figures 6c and 6d_).

Evaluation of the magnetic fabric parameter V (equation (2)), indicates inverse trends for siderite as compared with ankerite ore samples. Unheated siderite samples exhibit a prolate fabric, whereas ankerite samples are oblate. After heating, siderite samples become progressively more oblate, whereas ankerite samples become highly prolate (Figure 7).

DISCUSSION

The question we have raised, "Are the iron carbonate minerals, ankerite and ferroan dolomite, like siderite, important in paleomagnetism?", should be answered with an unqualified, "yes." It is clear from an examination of our thermal data that stabilities of ankerite and siderite are

similar. Our XRD data show that during thermal demagnetization, ankerite like siderite, breaks down to form maghemite [B.B. Ellwood et al., submitted manuscript, 1988], thus producing large RM and susceptibility increases (Figures 5-7). (Other minerals, such as pyrite, show similar changes and are the subject of ongoing ex- periments at the University of Texas at Arlington CUTA).) The magnitude of change is much less for ankerite, because significantly less iron is incorporated in the ankerite crystal

lattice than in the siderite lattice, with less iron available to form the iron oxides. Demagnetization experiments on siderite and ankerite ore samples indicate that thermal demagnetization may be effective below 250øC, but at higher temperatures these samples exhibit increasing moments (Figure 5). This new moment shows no between sample directional consistency, and intensities are very large after heating above 300oC. Therefore we infer that it is a secondary chemical RM (CRM) which was produced within the thermal demagnetizer and results from nucleation of magnetite or maghemite in the presence of internal magnetic fields present within each specimen.

Siderite is so unstable that it spontaneously deteriorates upon exposure in air to form magnetite or maghemite [Seguin, 1966], thus dramatically changing the magnetic properties of the sample [Ellwood et al., 1986, also submitted manuscript, 1988]. Pan of the question con- cerning the importance of ankerite in paleomagnetism deals with the weathering characteristics of the mineral. DTA and TGA analyses indicate that ankerite and siderite have comparable thermal stabilities, and both minerals are less stable than either calcite or dolomite [e.g., Smykatz-Kloss, 1974]. Because carbonates may alter relatively rapidly, ankerite and ferroan dolomite, with low thermal stabilities, like siderite, can be expected to alter rapidly if exposed in outcrop, with the resulting oxidation products being an iron oxide or oxyhydroxide. Because of the relatively few

11o

(a) SIDERITE •

90

t-so

"- TGA 7o 525.34oc '- .....

, 60 ' ' ' ' ooo

o 12o

•, _ (b) ANKERITE m 4 -•-- '• ,, DTA / Z 100 III - ",

m -2-

-- C3 80

n- -3

___ uJ -4 786.1

,,, - 845.48oc -

-5 .... ' 40 460 ooo

TEMPERATURE (øC)

Fig. 4. DTA and TGA curves for (,a) siderite and 02) ankerite. The TGA data are shown in terms of integral weight loss.

Ellwood et al.' Ankerite and Ferroan Dolomite 7327

I I

I

I

SIDERITE

TE (ORE)

ANKERITE (AM 4366)

ANKERITE (GLENROSE)

I I I o 1 oo 200 300 400 500

TEMPERATURE (øC)

•M

3xlo -2

lxlO -2

_ 8xlo-3

_ 6xlo-3

- 4xlo -3

_ 2xlo -3

_ 9xlo-4

_ 7xlo-4

_ 5xlO-•

_ 3xlo-•

_ lxlO-•

_ 8xlo-s

_ 6xlo-s

- 4xlo -s

- 2xlO-S

9xlo -e

x

Fig. 5. Susceptibility (Z) and total RM (M) data for a single sidefire and three ankedte samples, coarse crystalline ore, cement, and fine crystalline samples, which have been incrementally heated from 0 to 450 ø or 500øC, respectively. At these temperatures, several of the samples disintegrated.

7328 Ellwood et al.: Ankerite and Ferroan Dolomite

N N

350 i

a b

Ankerite Siderite

N N

400

c d

Calcite Dolomite

Fig. 6. Lower hemisphere, equal-area plot of anisotropy of magnetic (AMS) data for (,.a.) ankerite, (]2) siderite, •) calcite, and d(• dolomite samples. The AMS was measured for each sample, the sample was then heated and the AMS remeasured after cooling to room temperature.

Temperature of heating is labeled. Squares, Z 1; triangles, Z2; circles, •3-

entirely different; the oxidized siderite exhibiting oblate characteristics while ankerite samples exhibit a prolate character (Figure 7). This difference may be due to the production of varying amounts of very fine iron oxide grains, which may be distributed differently in the crystal lattice of each mineral examined. Ankerite, with much less iron than siderite but with other elemental contaminants

(e.g., Mg and possibly Mn), incorporates these constituents in its new oxidation-formed crystalline structure. Siderite oxidation, on the other hand, probably yields a much purer and therefore more uniform or ordered iron oxide residual. Changes in AMS axial patterns occur at slightly higher temperatures in ankerite (Figure 6a_) than in siderite (Figure 6.b_). We interpret this difference to result from the slightly slower release during heating of iron, indicated by the ankerite TGA data (Figure 4b, -470 ø- 650øC), and a corresponding slower buildup of iron oxides in the ankerite sample (Figure 61t). The important diag- nostic for paleomagnetists is the change in magnetic fabric produced by iron carbonates, that can be attributed to production of new ferrimagnetic phases and the corre- sponding remanent and induced magnetic changes exhibited by these samples

Trends in the fabric patterns during the heating experiments on the noniron carbonates, calcite and

8O

7O

SIDERITE

pure ankerite samples we were able to obtain and the short- term laboratory experiments we were able to perform, we saw no evidence in ankerite samples of the spontaneous RM changes already associated with siderite JEllwood et al., 1986, also submitted manuscript, 1988]. However, we conclude that these changes are probable and may, in some case studies, explain the presence of secondary oxy- hydroxides or hematite that has been attributed to dedolomitization of ferroan dolomite [Elmore et al., 1985].

Magnetic fabric. The magnetic fabric for samples of ankerite and siderite exhibits some striking similarities. For example, the AMS fabric diagrams in Figure 6 show that for samples that have not been heated, the maximum (Z 1) principal axis is normal to the long axis of the paleomagnetic core. Z l is also normal to the bedding or sedimentary foliation where a sedi-mentary fabric orien- tation can be identified. We postulate that the magnetic fabric in these samples is controlled either by an AMS sensitivity to sample shape when paramagnetic materials or very fine ferrimagnetic particles are the principal contributors to the AMS JEllwood, 1979] (Figure 8), or to a dominant magnetocrystalline anisotropy in the samples examined. Increasing temperature and the corresponding progressive oxidation of siderite and ankerite generates new, ferrimagnetic phases which alter the fabric patterns in individual samples (Figures 6a and 6b_). This altered pattern appears to be similar for siderite and ankerite, but the magnetic fabric shapes (elongated as opposed to flattened) for individual samples of these two minerals are

6O

V 40

3o

ANKERITE

2O

10

i I I I

0 100 200 300 400 500

ToC

Fig. 7. Magnetic fabric shape data (Graham's V [Graham, 1966] for

thermally treated ankerite and siderite samples. When V < 45 ø, the fabric is prolate; when V > 45 ø, the fabric is oblate.

Ellwood et al.' Ankerite and Ferroan Dolomite 7329

dolomite, are different from that for siderite and ankerite. For example, the fabric pattern for the calcite sample in Figure 6c• exhibits a great circle distribution of ;t 1 and Z2 principal axes, which we attribute to a crystallographic control in the calcite on the Z3 AMS orientations, while Z1 and Z2 are essentially unconstrained. The magnetic fabric in the dolomite sample proved to be very stable, because no changes in fabric patterns were observed with heating to 600øC (Figure 6d_). (Demagnetization experiments show that the dolomite samples analyzed in this study exhibit a stable RM, probably carried in very fine grained magnetite. Further analyses of these samples is currently underway at UTA.) While all the fabric patterns at low temperatures (<200øC) are similar, the striking differences observed at higher temperatures in siderite and ankerite are produced by the development of new magnetic phases in samples containing these minerals.

M6ssbauer effect studies of ankerite. The M0ssbauer effect proves very helpful in the study of ankerite and siderite in rocks. Specifically, the isomer shift, •, which is a direct measure of the s-electron density at the iron 57 nucleus varies in a predictable fashion with oxidation state and coordination number. The quad-rupole interaction, which results from the interaction of the iron 57 nuclear

quad-rupole moment with the electric field gradients present in iron containing minerals, is an effective measure of the distortion of the electronic environment from cubic

symmetry [Long, 1986]. Less well known, but of equal importance in this work, is the use of the M0ssbauer effect to study the lattice dynamics and stability of a material [Herber, 1984]. The minerals recoil-free fraction, as determined by the area of the M6ssbauer absorption spectrum, and its temperature dependence may be used to determine the effective M6ssbauer temperature, 0 M, and the effective iron 57 absorber mass. As discussed below, these quantifies are useful in describing the relative thermal properties of different minerals.

M6ssbauer spectral data illustrate the strong similarity between the Erzberg ankerite, Ca(Fe0.53Mg0.47)(CO3)2, and the sample, Ca(Ca0.1Fe0.3Mg0.5Mn0.1)(CO3)2, studied by DeGrave and Vochten [1985]. An analysis of the temperature depend-ence of the spectral results obtained between 80 ø and 300øK on the latter ank-erite sample revealed an isomer shift with a slope of-5.97 x 10 -4 (mm/s)/øK and an intercept of 1.412 mm/s at 0øK. Hence, as expected, the bonding in the two com-pounds is virtu- ally identical as indicated by the comparable isomer shift values given in Table 2. The site symmetry for the iron ions in the two ankerite samples is also virtually identical. The results for Ca(Ca0.1Fe0.3Mg0.5Mn0.1)(CO3)2 between 80 ø and 300øK by DeGrave and Vochten [1985] yield a linear temperature dependence to the quadrupole splitting with a slope of-1.91 x 10 -3 (mm/s)/øK and an intercept of 2.049 mm/s; values that are virtually identical to those found for our sample (Table 2). We were surprised to find that these data indicate that both ankerite samples have very similar electronic environments to that found in siderite. This follows from the similarities in the

magnitude and temperature dependence of the isomer shift for siderite as illustrated in Figure 2 and in Tables 1 and 2.

1.2

1.0

0.8

0.6

-381•m 0.4

0.2 • I I I I I I I I 0 10 20 30 40 50 60 70 80 90

Inclination

Fig. 8. Changes in Z 1 inclination which correspond to changing the length/diameter ratio for two samples constructed in the laboratory which are composed of 0.5 •tm and 20-38 gxn magnetite. The data are redrawn from Ellwood [1979] and illustrate the extreme sensitivity to sample shape when magnetic grain size is very small.

More surprising, however, is the stronger temperature dependence of the quad-rupole splitting, aEQ in ankerite, as com-pared with siderite, which has higher splittings. The quadrupole splitting in siderite has been the subject of several experimental and theoretical studies (see references olDeGrave and Vochten [1985] and Price [1978]). Simple crystal field models do not reproduce the essentially linear temperature dependence of the quad-rupole interaction in siderite. _Price [1978] has proposed the presence of an or- bit-lattice interaction, resulting from a temperature dependent admixture of the ground state, 5Eg•, and the excited state 5Al__g, crystal field states in the trigonal en- vironment. Apparently, this admixture produces an electric field gradient at the iron (II) site that decreases more rapidly than predicted by a simple crystal field model. As proposed by DeGrave and Vochten [1985], it appears that the larger ionic radius of the calcium (II) ion, and the resulting larger unit cell in ankerite, as compared to siderite, enhances the vibronic coupling in ankerite and yields a larger decrease in the quadrupole interaction with increasing temperature. It is interesting to note that the line width of each component of the quadrupole doublet in our ankerite sample also increases with decreasing temperature in the rather unusual fashion observed by DeGrave and Vochten [1985].

By using both the temperature dependence of the isomer shift and of the spectral absorption area, it is possible, as discussed by Herber [1984], to calculate both an effective absorber mass Meff for the iron 57 site and an effective

M6ssbauer temperature 0M, which can be considered analogous to the Debye temperature in a pure element. The resulting values for siderite and ankerite are given in Table 2. The linear plots of In A versus temperature, shown in Figure 2, indicates that the linear approximation used for these calculations is valid. Because the values of

7330 Ellwood et al.' Ankerite and Ferroan Dolomite

M__.eff are somewhat larger than 57, it appears that the recoiling nuclide is effectively bonded in the oxide lattice through covalent interactions. The resulting 0M values are typical for oxides of this type and the value of 328øK for our ankerite sample closely approximates the value of approximately 300øK obtained by DeGrave and Vochten [1985], especially when it is realized that the values were obtained by using very different methods. It is difficult to determine the accuracy of the 0M values, but we estimate that they are valid to approximately +/- 5øK, indicating that the value is somewhat higher in siderite than in ankerite. The higher value for siderite seems unexpected in view of the lower temperature of thermal decomposition of siderite (Figure 4• as compared to ankerite (Figure 4.b_). However, the higher value found for siderite is probably a reflection of the smaller unit cell volume in siderite as

compared to ankerite. Finally, it is interesting to consider whether our

ankerite sample could contain a small amount of siderite. An analysis of the ankerite MOssbauer spectra, which included a component with hyperfine parameters constrained to the values measured for siderite (Table 1) indicated that the sample may contain a small amount of a component resembling siderite. However, it was not necessary to add this component to obtain a good fit to the experimental data, as is indicated by the solid line fit shown in Figure 2. Hence it appears that the effect of the siderite component, if present, in our ankerite MOssbauer spectra is insignificant.

The effective MOssbauer temperature 0M__ is slightly lower in ankerite than in siderite (Table 2), suggesting that the iron in the ankerite crystal lattice is slightly less tightly bound than is the iron in siderite. This indicates that ankerite is at least as unstable as siderite (also observed in the magnetic data (Figures 5 and 6)) and therefore equally or possibly slightly more susceptible to oxidation in air with the spontaneous production of secondary iron oxides. The major difference between the two minerals is in the larger amount of iron, available for the production of sec- ondary iron oxides, contained in the siderite crystal lattice.

Strawn Sand, Archer County Texas. The Strawn Sand in north Texas is an important oil reservoir rock with production at more than 1 billion barrels. Secondary re- covery efforts are now underway to enhance production if possible, and two cores drilled from the unit are available for study. We have examined the cementing agent in one core and have determined, using XRD analysis, that the cementing material is ankerite (Figure 1). Since chlorite and other iron-containing minerals are present in the core, the exact iron/magnesium ration is difficult to determine, but the XRD data indicate potentially the highest iron/magnesium ratio ever reported for ankerite. Total weight percent iron in some samples is higher than 30%.

The magnetic properties of the S trawn ankerite cement are very similar to the Erzberg ankerite ore sample, with the exception that initial g and RM vales are lower (AM4366, Figure 5). However, heating of these samples above 300oc shows the dramatic increases in g and RM

seen in the ore samples and is attributed to conversion of the ankerite to maghemite. The implications of the presence of high iron ankerite cements for s•ondary re-

covery techniques are currently being examined. Since ankerite readily oxidizes to form iron oxides, these secondary minerals may precipitate in pore spaces in the reservoir, potentially reducing production. This effect has been observed during laboratory experiments on siderite (J. Cooper, personal communication, 1988).

Ankerite filling dinosaur footprints. The dinosaur tracks exposed in the bedrock of the Paluxy River at Dinosaur Valley State Park near Glen Rose, Texas, represent some of the best preserved tracks in the world. The trackways are in the lower member of the Lower Cretaceous Glen Rose

Formation at the Taylor site, --1.5 km upstream from the "main" track site in the park. The tracks were formed as bipedal dinosaurs walked across a muddy tidal surface at low tide, probably in search of food [Farlow, 1987; Hastings, 1987]. The excellent preservation and detail of the tracks suggests that the tidal lime muds were relatively dry and quickly filled with terrigenous, silty lime-rich muds which were washed over the site by flooding from inland storms. This grey sediment, now lithified and coated with a layer of organic-rich material, rams brown when scrubbed clean and exposed to the oxygenareal waters of the Paluxy fiver. Our XRD data show that this track-filling material is ankerite (Figure 1), while the footprints were formed in a layer of limestone. The coloration produced by oxidation of ankerite outlines the derailed dinosaurian footprint features unique to this locality. Only when the iron oxide stain appears after the rock is cleaned is it possible to see the long talon marks extending from the dinosaur's toes.

The magnetic properties of the ankerite filling the dinosaur footprints show different trends than do all other ankerite samples examined. The Glen Rose anker-ite is very fine crystalline material and with heating above 300oC oxidizes to mag-hemite, with the corresponding large increases in g and RM (Figure 5) observed for other ankerite samples. Then, further heating and oxidation converts the maghemite to hematite (identified using M0ssbauer effect data), with a corresponding large decrease in magnetic property magnitudes (Figure 5). This hematite develops during oxidation of the sediment filling some of the dinosaur tracks at Glen Rose and explains why these footprints appear to rust when they are exposed in the Paluxy fiver bottom.

CONCLUSIONS

The iron carbonate, ankerite, like siderite, rapidly oxidizes in air to produce the iron oxides maghemite and hematite, which than can dominate the magnetization of paleomagnetic samples. However, ankerite has signifi- canfly less iron available in its crystal lattice for the production of iron oxides than does siderite. Therefore the effects on the magnetic properties of rocks containing ankerite (or ferroan dolomite) may take longer to develop in samples than equivalent effects produced on the magnetic properties of samples by oxidation of siderite. Oxidation of very fine grained ankerite to hematite, which is magnetically stable, may produce a secondary moment in samples that can not be remaved using standard demagnetization techniques. However, the magnitude of these secondary magnetic properties appears to be small and may not dominate the moment in paleomagnetic samples.

Ellwood et al.: Ankerite and Ferroan Dolomite 7331

This would be especially true if the grain size is so small that the behavior is essentially superparamagnetic.

Acknowledgements. The Mtssbauer effect work and the thermal analysis experiments were supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society through grant 18202-AC3, and the DuPont Company, respectively. We would like to thank E. DeGrave for supplying numerical data from their study of ankerite. James E. Cooper is gratefully acknowledged for his helpful comments.

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(Received May 20, 1988; revised November 14, 1988; accepted January 2, 1989.)