Nature of dioctahedral micas in Spanish red soils · Nature of dioctahedral micas in Spanish red soils ... species in the surface of the crust are phengites, ... (diameter), condenser

Clay Minerals (1997) 32, 107-121

Nature of dioctahedral micas in Spanish red soils

J. M. M A R T I N - G A R C I A , G. D E L G A D O * , M. S A N C H E Z - M A R A N O N l , J. F. P A R R A G A * AND R. D E L G A D O *

Departamento de Geologla, Facultad de Ciencias Experimentales, Universidad de Ja~n, Paraje Las Lagunillas, 23071, Ja~n, Spain, * Departamento de Edafologia y Qufmica Agricola, Facultad de Farmacia, Universidad de Granada,

Campus Cartuja, 18071, Granada, Spain, t Departamento de Edafologia y Qulmica Agrlcola, Universidad de Alrneria, 04120, Almeria, Spain

(Received 14 July 1995; revised 29 March 1996)

ABSTRACT: Structural formulae and other crystallochemical parameters were used to study different species of dioctahedral micas in clay and coarse gravel fractions of horizons from a red soil (Ultic Haploxeralf) in southern Spain. Mineralogical analyses using X-ray powder diffraction, and measurements of the b0 parameter revealed dioctahedral micas, illite and paragonite. Structural formulae established from electron microprobe analysis and energy dispersive X-ray analysis showed the illites to be K mica related in elemental composition and structure to muscovite and phengite. The paragonites were found to be closer to ideal mica. Structural formulae for Na-K dioctahedral micas were obtained with crystallochemical characteristics intermediate between those of Na micas and K micas. The possibilty of these micas representing individual mineral phases or intergrowths of Na and K micas is discussed, In the soil profile, micas from the Bt horizon showed the largest crystallochemical changes induced by pedogenesis.

Micaceous phyllosilicates are frequent in soils; the US Soil Taxonomy (Soil Survey Staff, 1975, 1994) distinguishes the micaceous, illitic and glauconitic soil families. Although many studies have noted the presence of micas in soils (Allen & Hajek, 1989), few have identified the exact mineral species, because of the difficulties in determining the composition and structural formulae of these minerals in soil.

Most of the micas identified in soils thus far belong to the subgroup of dioctahedral micas (trivalent cations -R 3+- in the octahedral sheet: Bailey, 1980) and are inherited or weakly transformed (Graf von Reichenbach & Rich, 1975). Muscovite has been identified in the sand and silt fractions, and is believed to be mostly inherited (Calvert et al., 1980). The clay fraction often contains illite, a sedimentary mica of small particle size (Weaver, 1989) and dioctahedral nature (Allen & Hajek, 1989) which can be distinguished

as a separate member of the group of true micas (Martin et al., 1991). Many investigators have described illites in soils (Pefia & Torrent, 1984; Srodofi & Eberl, 1984; Delgado et al., 1990, 1994; Colombo & Torrent, 1991; Laird & Nater, 1993). The dioctahedral mica paragonite has also been found in different soil fractions (Delgado et al., 1990; Garcfa-Gonzfilez & Aragoneses, 1990; Levy & Graham, 1993).

Other species of dioctahedral micas such as phengite, celadonite and glauconite exist as rock- forming minerals which can subsequently form part of the inherited phases of soils. The most common species in the surface of the crust are phengites, which are typical of metamorphic rocks (Velde, 1985; Shau et al., 1991; Jong et al., 1992; Guidotti et al., 1994b).

In general, micas of the dioctahedral subgroup are more stable than trioctahedral minerals in environments subjected to supergene alterations

�9 1997 The Mineralogical Society

108

(such as pedologic environments). This is because the higher occupancy in the octahedral sheet of trioctahedral minerals increases the repulsion between cations, and because the H § in OH- are oriented toward the interlayer K + (Serratosa & Bradley, 1958; Fanning et al., 1989).

In the present study we investigated the presence of different species of dioctahedral mica in soils by calculating structural formulae and analysing crystallochemical parameters.

J. M. Martin-Garcia et al.

�9 Sampting zone

A @s,ERRA NEVADA ?spai o = , , . , .....

, :~ . ; - " �9 ."

'Lf- Mediterranea~ Sea M A T E R I A L S A N D M E T H O D S

We selected a profile of a fersiallitic red soil classified by the US Soil Taxonomy (Soil Survey Staff, 1975, 1994) as an Ultic Haploxeralf, coarse- loamy over loamy-skeletal, micaceous mesic. The profile was sampled in the Sierra Nevada mountains in southeastern Spain (Table 1, Fig. 1). The soil parent material consists of slope deposits from metamorphic rock showing a high to moderate degree of metamorphism (greenschist facies) rich in micas, including graphitaceous mica-schists, quartzites and micaceous quartzites (Puga & Dfaz de Federico, 1976). The main morphological and analytical features of the profile are summarized in Table 1 (Martfn-Garcfa, 1994).

We examined the clay fraction (<2 ~m) and the coarse gravel fraction (>8 mm) to cover the extremes in the spectrum of grain sizes in the mineral fraction of the soil. The clay fraction was separated in suspension, and coarse gravel was

FIG. I. Location of the sampling zone in the Sierra Nevada mountains, southeastern Spain.

sieved out and washed in water to remove finer fractions that might have adhered to the surface.

Mineralogical composition was determined by X-ray diffraction (XRD) with a Philips PW 1730 diffractometer using Ni-filtered radiation (Cu-K~) at 35 kV and 15 mA. Samples of disoriented crystalline powder (all fractions) were prepared by using a holder filled from the side (Niskanen, 1964) to avoid the tendency of phyllosilicates to display preferred orientation of 00l reflections, In addition, oriented aggregates (clay fraction) were prepared by sedimentation and drying on a glass slide. To identify the composition of oriented specimens, solvation treatments with ethylene glycol and dimethylsulfoxide were used (Gonzfilez-Garcfa &

TABLE 1. Main morphological and analytical characteristics of the soil.

Sand Silt Clay Gravel (%) OC pH CEC Sat. Horizon Depth (cm) (%) (%) (%) Total Coarse (%) (cmol+/kg) (%)

Ap 0 --5/10 68.7 21,0 10.3 37 40 0.39 6.9 4.5 55.3 AB 5/10 -12/17 60.5 22.5 17.0 37 47 0.37 6.5 7.3 57.5 Bt 12/17 -30/50 54.8 21.8 23.4 51 62 0.26 5.7 8.5 51.8 C1 >30/50 61.7 19.4 18.9 69 83 0.24 5.6 8.3 49.4 C2 >30/50 51.5 14.5 34.0 60 79 0.25 5.5 11.6 65.5

All values are referred to 100% fine earth (105~ Particle size was determined in accordance with the Soil Conservation Service (1972). Organic carbon (OC) was measured using the method of Kononova (1981). Cation exchange capacity (CEC) and base saturation (Sat.) were measured with AcNH4 (pH 7) (Soil Conservation Service, 1972). pH was measured in a 1:1, soil:water suspension. Location: Loma de la Cuna de los Cuartos (Sierra Nevada, Spain); UTM coordinates: 30SVG645117; Physiographic Position: mountainous slope, 1410 m elevation; Slope: 23%; Use: abandoned cropland.

Dioctahedral micas in soils 109

S~inchez-Camazano, 1968; Brown & Brindley z 1980). In all cases, semi-quant i ta t ive mineral analysis (XRD) was carried out with intensity factors (Schultz, 1964; Klug & Alexander, 1976). Phyllosilicate minerals in the clay fraction were quantified in oriented aggregates with ethylene glycol solvation. The following intensity factors were used (Schultz, 1964; Martfn Pozas et al.,

1969; Barahona, 1974; and Delgado et aI., 1982): (1) Disoriented crystalline powder: quartz, d = 0.426 nm, factor = 0.27; feldspars, d = 0 .320-0.325 nm, factor = 1; total clay minerals, d = 0.445 nm, factor = 0.10; goethite and hematite, d = 0.269 nm, factor = 0.24; chlorite, d = 0.705 nm, factor = 1. (2) Oriented aggregates: illite, d = 1.0 nm, factor = 1.0; paragonite, d = 0.96 rim, factor = 2.0; chlorite, d = 0.705 nm, factor = 2.0; kaolinite, d = 0.720 nm, factor = 2.0; smectite, d = 1.70 nm, factor = 4.0.

The mean dimension of the b0 parameter in micas was measured from the 060 reflection (approximately equal to 0.150 nm) in disoriented powder samples (scan speed: 0.5 ~ 20/min). Quartz in the sample was used as an internal standard, and when necessary, high purity quartz was added.

Micas in the coarse gravel f ract ion were e x a m i n e d with e lec t ron mic roprobe analys is (EMPA) (Camebax SX-50 WDS), with a probe current of 20 nA and a 7 p.m beam. Gravel particles were hardened with a mixture of Stratyl AI-100 resin, acetone, catalyser, and polymerizing agent, then cut, polished and coated with graphite. This process was also used to prepare thin-sections of gravel which were examined to select appropriate specimens for EMPA.

Micas in coarse gravel were also observed in a Hi tachi S-510 scann ing e lec t ron mic roscope (SEM) run at an acceleration voltage of 25 kV. Fresh-broken faces of gravel f ragments were mounted on the stub with silver paint, coated with gold (current 5 0 - 1 0 0 A) with a Sempred 2 appa ra tus in two o r i e n t a t i o n s ( 2 0 - 3 0 ~ ) as recommended by Bohor and Hughes (1971) to enhance image quality. Relative pedogenic evolution of the gravel was estimated by sampling and observ ing f ragments of una l te red rock from outcrops near the soil profile.

Clay samples representative of different soil horizons were examined with transmission electron microscopy (TEM) using a Philips CM-20 microscope coupled to an X-ray microanalysis (EDX) system (EDAX DX4) operating at 200 kV, slit width 40 gm, to obtain information on particle morphology, structural formula, and selected area e lect ron diffract ion (SAED) (paral lel b * - c * ) patterns of mica. The [001] crystallographic axis of mica was oriented parallel to the plane of section for microanalysis. Scanning-transmission electron microscopy was used with a 1 gm x l Ixm window, spot size 100A (diameter), condenser aperture 50 lam (low background), live time 200 s, dead time <5%, manual background, 300 and 500 cps.

The structural formulae of the micas analysed with EMPA and EDX were calculated on the basis of a cation charge of 22. Iron was always assumed to be Fe 3+, because of the difficulty of distin- guishing between Fe 2§ and Fe 3+ in white micas

(Guidotti, 1984).

TABLE 2. Semi-quantitative mineralogical analysis by X-ray diffractometry.

Clay (<2 i.tm) Tect. Iron Phyllosilicates

Horizon (%) forms (%) Total I1 Par Ka Chl Sm Others

Coarse gravel (>8 mm) Tect. Iron Phyll. (%) forms (%) (%)

Ap 16 <5 84 62 6 9 7 <5 Int AB 14 5 81 59 <5 14 8 <5 Int Bt 8 7 85 58 8 12 7 <5 Int C1 9 8 83 54 7 15 7 <5 Int C2 8 l0 82 49 <5 25 8 <5 Int

64 <5 36 62 <5 38 49 <5 51 54 <5 46 59 <5 41

Tect.= tectosilicates (quartz and feldspars); Iron forms = hematite and kaolinite; Chl = chlorite; Sm = smectite; Int = interstratified minerals; micas, interstratified minerals, chlorite and kaolinite.

goethite; I1 = illite; Par = paragonite; Ka = Phyll. (phyllosilicates) in coarse gravel =

110 J.M. Martfn-Garcfa et al.

R E S U L T S A N D D I S C U S S I O N

Semiquantitative mineralogical analysis (XRD)

The most abundant minerals in the clay fraction were phyllosilicates (illite, paragonite, interstratified phases, kaolinite, chlorite and smectite), accounting for 81-85% of the total composition (Table 2). These were followed by tectosilicates (quartz and feldspar) and Fe phases (goethite and hematite). Illite was the most abundant mineral species in the clay fraction. The value of bo (Table 6) demon- strated the dioctahedral nature of this mica. Interstratified phases were detected by XRD as a wide band between 1.1 and 1.2 nm. Accurate characterization and quantification is not possible because their scarcity.

In coarse gravel, tectosilicates (mainly quartz) were the most abundant minerals (except in the Bt horizon), followed in decreasing order by phyllosilicates and the Fe-rich minerals goethite and hematite. Phyllosilicates consisted of micas and smaller amounts of interstratified phases, chlorite and kaolinite.

In the clay and gravel fractions, the appearance of two relatively well-resolved reflections at approximately 1.0 nm and 0.96 nm corroborated the presence of K and Na micas (Fig. 2).

The major portion of the phyllosilicates in the soil was inherited from parent material (i.e. micas), as deduced from their presence in mica-schists and quartzites typical of the Sierra Nevada region (Puga, 1976). Transformed phases were represented by inters t ra t i f ied phases and smectite; and neoformed phases were represented by kaolinite. The presence of kaolinite in the coarse gravel fraction resulted from pedologic evolution in this fraction (Martfn-Garcfa, 1994), as kaolini te becomes proportionately more abundant in red soils as they develop (Delgado et al., 1990; Delgado et al., 1994).

Electron microprobe analysis (EMPA) of the coarse gravel fraction

Ninety seven randomly chosen points in the micaeeous bands of coarse gravel were analysed. The gravel consisted mainly of microfolded packets of mica with 001 planes on the surface of schistosities and packets of quartz appearing as small crystals with indented edges (Fig. 3). Tables 3

E c-

O o.

~ E

~, ~ .~

.o_ E

Z

I

! clay

L gravel

I r

8 10

FIG. 2. Powder X-ray diffraction pattern of the clay and coarse gravel fractions in region 5 -10 ~ 2h (Cu-Ka).

and 4 give the mean values found with EMPA for the elemental composition and structural formulae of micas; Fig. 4 illustrates the relation between Na* (Na* = 100 • Na/(Na + K)) and Fm (Fm = F e + Mg) in the 97 points sampled. Values for Na* and Fm were calculated according to Guidotti et al. (1989), who established that the Na* ratio is ~< 15 in K micas (similar to muscovite). For Na micas, Shau et al. (1991) reported a value of Na* ~>80. In our material, 60 sampling points yielded a value of Na* ~<15, eight gave a value of />80, and 29 were of intermediate composition (Na-K micas).

The mean values of Na* in K micas ranged from 9.23 to 12.66 (Na* ~< 15) (Table 3). They contained less tetrahedral AI (mean values from 0.69 to 0.81) than muscovites (theoretical value of 1.00). In the octahedral sheet, A13+ atoms of ideal muscovite were replaced by Fe 3+, Mg 2+, and (to a lesser


FIG. 3. Optical micrograph of a thin-section of a coarse gravel fragment from the Bt horizon, showing packets of mica. Magnification 115 x, parallel polarizers. Mc - Mica, Qz - Quartz.

extent) Ti 4+, giving rise to structural formulae with an octahedral charge other than zero. The value of Fm ranged from 0.25 (C2 horizon) to 0.36 (Bt horizon), The charge balance showed a relatively low layer charge (between 0.71 in the Bt horizon and 0.87 in the AB horizon), requiring a lesser level of interlayer cations than in theoretical muscovite. These features indicate that micas in the coarse gravel evolved from the ideal mica, and showed micas in the Bt horizon to be the most changed by pedogenesis, with a layer charge of 0.71 and an interlayer cation content of 0.67. This horizon most clearly illustrated pedogenic processes (e.g. illuvia- tion, alteration, reddening) (Martfn-Garcfa, 1994).

Sodium micas (mean Na* = 83.91, >80) contained more A1 (in both tetrahedral and octahedral sites) and larger amounts of interlayer cations than K micas, but slightly less than the theoretical values for paragonite (Table 3). In contrast, octahedral Fe and Mg contents were lower than in K micas (Fm <0.15), and somewhat higher than in ideal "paragonite. Layer charge was greater than in K micas because of the lesser extent of isomorphic substitution.

The 29 micas with a Na-K interlayer composition (Table 4) displayed crystallochemical characteristics that were intermediate between those of Na and K micas (Table 3). These minerals raise some

interesting points. (1) XRD patterns contain diagnostic peaks only of Na and K micas (Fig. 2). (2) The composition of intermediate micas places them within the range of inmiscibilities of K and Na micas, i.e. a composition intermediate between the paragonite and muscovite limbs of the solvus (Guidotti et al., 1994a,b; Blencoe et al., 1994). These limbs correspond approximately to the thresholds we used (Fig. 4) to distinguish between K (Na* ~< 15) and Na micas (Na* />80). According to Guidotti et al. (1994a, b) and Blencoe et al.

(1994), the solvus limbs in metamorphic rocks depends on pressure and temperature conditions. Greenschist facies mica-schists, micaceous quartzites and quartzites in the Veleta nappe were formed at temperatures no higher than 450 ~ and pressures not surpassing 7 kbar (Puga & Dfaz de Federico, 1976). Under these conditions, intermediate micas fall within the solvus; even if temperatures surpassed 650 ~ they would remain unstable. Nevertheless, Li et al. (1994) have reported that there may exist genetic environments that give rise to micaceous species whose composition is intermediate (although metastable) within the solvus between muscovite and paragonite. However, regional metamorphism is not a feature of these environments. (3) Our electron probe microanalysis may have simultaneously estimated

112 J.M. Martin-Garc[a et al.

TABLE 3. Composition and structural formula of K and Na micas, determined with electron microprobe analysis in the coarse gravel fraction. All values are means.

Na-micas K-micas (Na* ~< 15) (n = 60) (Na*/> 80)

(n = 8) Horizon A p ( n = 19) AB (n= 14) B t ( n = 4 ) C1 (n=20) C 2 ( n = 3 ) Profile

mean on- 1 mean on_ 1 m6an o._1 mean o._ 1 mean a n - - 1 mean On_l

Si 47.69 1.29 46.45 0.83 49.19 0.57 46.87 1.44 46.76 1.30 44.64 0.82 A1 36.08 1.91 36.49 0.94 35.40 1.80 36.54 1.77 37.79 1.60 41.53 0.95 Fe 2.33 0.82 1.89 0.25 2.27 0.43 2.24 0.47 1.88 0.49 0.61 0.26 Mg 2.50 0.74 2.22 0.41 3.14 0.75 2.29 0.55 1.80 0.22 0.47 0.26 Ti 0.23 0.04 0.23 0.02 0.17 0.06 0.25 0.03 0.22 0.03 0.04 0.0l Ca 0.12 0.12 0.02 0.02 0,30 0.28 0.03 0.02 0.03 0.03 0.12 0.03 Na 1.11 0.44 1.63 0.25 0.83 0.67 1.32 0.27 1.46 0.16 10.59 1.13 K 9.94 0.95 11.07 0.24 8.70 1.84 10.46 0.95 10.06 0.96 2.00 0.53

Si TM 3.25 0.05 3.20 0.03 3.31 0.07 3.21 0.04 3.19 0.03 3.08 0.02 A1TM 0.75 0.05 0.80 0.03 0.69 0.07 0.79 0.04 0.81 0.03 0.92 0.02 A1 vl 1.71 0.07 1.72 0.04 1.69 0.05 1.71 0.07 1.77 0.03 1.94 0.03 Fe vI 0.16 0,05 0.13 0.02 0.15 0.03 0.15 0.04 0.13 0.04 0.04 0.02 M g.vl 0.17 0,05 0.15 0.04 0.21 0.05 0.16 0.04 0.12 0.02 0.03 0.02 Ti v~ 0.02 0,01 0.02 0.00 0.01 0.01 0.02 0.00 0.02 0.01 0.00 0.00 C a XlI 0.01 0.01 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.01 0.00 Na xn 0.08 0.03 0.11 0.02 0.06 0.04 0.09 0.02 0.10 0.01 0.73 0.08 K xn 0.68 0.06 0.76 0.02 0.59 0.12 0.72 0.06 0.69 0,05 0.14 0.03

x TM -0.75 0.05 -0.80 0.03 -0.69 0.07 -0.79 0.04 -0.81 0,03 -0.92 0.02 x v~ 0.03 0.09 -0.07 0.02 -0.02 0.11 -0.02 0.12 0.02 0,03 0.00 0.00 x -0.72 0.06 -0.87 0.03 -0.71 0.13 -0.81 0.08 -0.79 0.05 -0.92 0.05 x xn 0.78 0.07 0.87 0.02 0.69 0.12 0.81 0.06 0.79 0.06 0.89 0.06 R VI 2.06 0.03 2.02 0.01 2.06 0.05 2.04 0.03 2.04 0.02 2.01 0.02 R 3+v~ 1.87 0.04 1.85 0.02 1.84 0.02 1.86 0.06 1.90 0.02 1.98 0.02 R XlI 0.77 0.08 0.87 0.02 0.67 0.14 0.81 0.07 0.79 0.05 0.88 0.06 Na* 10.53 3.51 12.64 1.74 9.23 5.17 11.11 1.67 12.66 1.22 83.91 4.47 Fm 0.33 0.09 0.28 0.04 0.36 0.08 0.31 0.06 0.25 0.05 0.07 0.03

Elemental analyses are given in % number of atoms. Structural formula for half unit-cell. Na* = 100Na/(Na+K) and Fm = Fe + Mg (Guidotti et al., 1989). IV: tetrahedral; VI: octahedral; XII: interlayer; R: cations; 3+: trivalent; x: charge.

packets of intergrown K and Na mica, a problem mentioned by Shau et al. (1991).

The above discussion (points 1, 2 and 3) does not support the existence of intermediate Na-K micas as discrete minerals phases, but suggests instead the presence of intergrowths between the two types. The problem raised by Na-K mica is further examined in the section below on the clay fraction, where additional results are discussed.

The occurrence of three sets of EMPA results (K, Na and intermediate micas) is supported by the highly significant correlations between Fm and Na*

(Fig. 4): K micas: Fm = 0.524-0.020 Na*; n = 60; r = - 0 . 7 8 0 8 ; sig. 0.001; intermediate micas: Fm = 0 . 2 8 2 - 0 . 0 0 2 Na*; n = 29; r = -0 .5 9 5 3 ; sig. 0.001. In addition, these correlations corrobo- rate the results of Guidotti et al. (1992) and document the parallel decreases in Fm and Na*.

Crystallochemical evidence of the pedogenic changes of micas in the gravel fraction is corroborated by SEM observations of ultramicromor- phology (Fig. 5). Mica grains showed rounded edges with a film of pedogenic material insinuated between the schistosity planes, giving rise to voids and


TABLE 4. Composition and structural formula of Na K micas, determined with electron microprobe analysis in the coarse gravel fraction. All values are means.

Na-K micas (15<Na*<80) Horizon Ap (n = 10) AB (n = 8) C1 (n = 3) C2 (n = 8)

mean On_l mean on_ 1 mean on- 1 mean ~n- 1

Si 46.88 0.71 44.80 1.32 46.13 0.67 46.42 0.72 A1 38.42 1.99 38.80 1.44 39.77 1.22 39.66 1.28 Fe 1.40 0.31 1.98 1.01 1.17 0.28 0.99 0.25 Mg 1.84 0.48 1.52 0.68 1.11 0.34 1.35 0.38 Ti 0.13 0.04 0.17 0.01 0.13 0.01 0.15 0.04 Ca 0.16 0.06 0.02 0.02 0.12 0.01 0.09 0.04 Na 4.44 2.09 3.47 1.57 5.72 1.77 4.60 1.98 K 6.73 1.56 9.24 1.35 5.85 0.89 6.74 1.49

Si TM 3.20 0.04 3.10 0.04 3.15 0.04 3.17 0.04 AI TM 0.80 0.04 0.90 0.04 0.85 0.04 0.83 0.04 AI vl 1.82 0.05 1.78 0.08 1.87 0.04 1.88 0.03 Fe vl 0.10 0.02 0.14 0.07 0.08 0.02 0.07 0.02 MgVl 0.13 0.04 0.11 0.05 0.08 0.02 0.09 0.03 Ti vl 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.00 C a XlI 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.01 Na xn 0.30 0.13 0.24 0.11 0.39 0.11 0.31 0.13 K xn 0.46 0.11 0.64 0.09 0.40 0.06 0.46 0.11

x TM -0 .80 0.04 -0.90 0.04 -0.85 0.04 -0.83 0.04 x vl 0.06 0.02 0.02 0.05 0.05 0.01 0.07 0.03 x --0.74 0.05 -0.88 0.05 -0.80 0.04 -0.76 0.05

XlI x 0.78 0.05 0.88 0.05 0.81 0.05 0.79 0.06 R vl 2.06 0.02 2.04 0.04 2.04 0.01 2.05 0.02 R 3+vl 1.92 0.03 1.92 0.02 1.95 0.02 1.95 0.02 R XII 0.77 0.05 0.88 0.05 0.80 0.05 0.78 0.06 Na* 39.47 15.75 27.27 11.15 49.37 11.70 40.26 14.88 Fm 0.23 0.05 0.25 0.12 0.16 0.04 0.16 0.05

Elemental analyses are given in % number of atoms. Structural formula for half unit-cell. Na* = 100Na/(Na+K) and Fm = Fe + Mg (Guidotti et al., 1989). IV: tetrahedral; VI: octahedral; XII: interlayer; R: cations; 3+: trivalent; x: charge.

channels. These features were not seen in unaltered fragments of the same lithologic characteristics.

TEM-EDX analysis o f the clay fraction

Observations with conventional TEM of clay particles in sections oriented parallel to the c-axis showed that planar particles (mostly micas, Table 2) predominated, and submicrofolds were absent.

Micas in the clay fraction were difficult to characterize with EDX and SAED because of their small particle size (<2 I.tm). We therefore examined groups rather than single particles, which accounts

for the low number of points recorded (Table 5). We detected dioctahedral micas with Fe and Mg substitutions in the octahedral sheet and interlayer cation content (Na +, K +, Ca 2+) <1. The Na* values indicate that the clay micas are predominantly K and Na-K phases; the XRD detected Na mica (Table 2, Fig. 2) but this was not, however, detected by EDX

because of the small number of points sampled. The SAED analyses provided interesting data on

Na-K micas. Figure 6 shows the SAED pattern (orientation parallel to c*-b*) for a clay-sized particle in the Bt horizon; the pattern corresponds to point 1 (Table 5) with Na* = 68.67. In 02/spots,

114 J.M. Mardn-Garc[a et al.

,..,0,4

+ = 0 , 3

U .

E U .

0,2

0,1

0 ,6

y = 0 , 5 2 4 - O . 0 2 0 x

n = 60 ; �9 = - 0 , 7 8 0 8 ; * * * 0 , 5 , ~ ~.,

, ~ " I+~ +1

�9 :

"%. i,, + L'l

+ ,=o282ooo2x L

�9 + I

�9 \ + +

\

0 10 20 30 40 50 60 70 80 90 100

N a * [ I O O N a / ( N a + K ) ]

Ms= muscovite; Pg= paragonite

�9 K-mica +K-Na-mica �9 Na-mlca

+ Guidotti et al (1989) ++ Shau e ta l (1991)

F~G. 4. Relation between Na* and Fm of micas in the coarse gravel fraction.

and more notably, in 04/spots (Fig. 6b), splitting of the diffraction maxima revealed the presence of two phases (Brearley, 1990). These phases can be ascribed to Na mica (paragonite) and K mica (muscovite-phengite) on the basis of the periodicity of 0.96 and 1.0 nm, respectively. The SAED pattern shows that the mica is a two-layer polytype, although some degree of disorder in the stacking sequence cannot be ruled out as the 00l and non-00/ reflections (K =~ 3n) were somewhat broad and diffuse (Baxter et al., 1991).

These results for the clay fraction do not support the existence of intermediate Na-K micas as an individual mineral phase, but suggest instead the presence of intergrowths between Na and K micas, a hypothesis also supported by the results of EMPA of coarse gravel.

The bo p a r a m e t e r

Table 6 summarizes the mean bo values calculated from the 060 reflection in micas from the clay and coarse gravel fraction. These values ranged from 0.9010 to 0.9031 nm, and corre- sponded to dioctahedral micas (Marffn-Ramos, 1976; Griffen, 1992). The values of b0 in clay and gravel were similar, corroborating the fact that the mica in both fractions was similar in structure and composition. However, mean bo was slightly h igher in the clay fract ion (0.9026 nm vs.

0.9020 rim). In an earlier study of micas from 'internal' zones

of the Betic mountain range, Martfn-Ramos (1976) calculated bo for muscovites without octahedral substitutions as 0.8980-0.8986 nm. Guidotti et al


A

B

FIG. 5. Scanning electron micrographs showing the ultramicromorphological evolution of gravel. A - mica-schist in the coarse gravel fraction, horizon C1. B - unaltered rocky fragments.

(1989) calculated bo for 2M1 muscovites as 0.8993-0.8994 nm. Griffen (1992) reported the theoretical bo for paragonite (0,899 nm), muscovite (0.900 nm) and phengite (0.904 nm). On the basis of these reports, the micas examined may be paragonite, muscovite, and intermediate forms between muscovite and phengite. The mineral

species of the micas in our material will be discussed in detail below.

To check the internal consistency of the results obtained with different methods, we examined, in the gravel fraction, the correlation between bo (Table 6) and mean values for some crystallochemical parameters in K micas, the most

116 J. M. Mart fn-Garcfa et al.

TABLE 5. Composition and structural formula of micas determined with X-ray microanalysis in the clay fraction.

Point 1 (P.) 2 (P.) 3 (G.P.) 4 (G.P.) 5 (G.P.) Horizon Bt Bt C2 C2 C2

Si 44.79 42.00 42.49 44.48 43.83 A1 41.88 40.25 41.10 39.84 37.75 Fe 0.51 2.34 2.74 1.90 3.49 Mg 0.22 2.93 2.24 2.46 3.12 Ti 0.00 0.00 0.00 0.00 0.17 Ca 0.55 0.14 0.29 0.57 0.13 Na 8.24 2.22 6.31 1.32 1.29 K 3.80 10.11 4.84 9.43 10.22

Si TM 3.08 2.94 2.94 3.06 3.03 AI TM 0.92 1.06 1.06 0.94 0.97 AI vI 1.96 1.76 1.79 1.80 1.64 Fe vl 0.04 0.16 0.19 0.13 0.24 M gVl 0.02 0.21 0.16 0.17 0.22 Ti vI 0.00 0.00 0.00 0.00 0.01 Ca xn 0.04 0.01 0.02 0.04 0.01 Na xIl 0.57 0.16 0.44 0.09 0.09 K XII 0.26 0.71 0.34 0.65 0.71

x TM -0.92 - 1.06 - 1.06 -0.94 -0.97 x vI 0.04 0.18 0.26 0.13 0.12 x -0.88 -0.88 -0.80 -0.81 -0.85 x xn 0.91 0.89 0.82 0.82 0.82 R w 2.02 2.13 2.14 2.10 2.11 R 3+vl 2.00 1.92 1.98 1.93 1.88 R XlI 0.87 0.88 0.80 0.78 0.81 Na* 68.67 18.39 56.41 12.16 11.25 Fm 0.06 0.37 0.35 0.30 0.46

Elemental analyses are given in % number of atoms. Structural formula for half unit-cell. G.P.: Groups of particles; P.: Single particle. Na* = 100Na/(Na+K) and F m = Fe + Mg (Guidotti et al., 1989). IV: tetrahedral; VI: octahedral; XII: interlayer; R: cations; 3+: trivalent; x: charge.

abundant phase of micas (EMPA, Table 3). Although only five data points were available (mean values of each of five soil horizons), we obtained significant correlations at 0.01 and 0.05 (Table 7). According to Marffn-Ramos et al. (1989), Guidotti et al. (1989, 1992) and Smoliar-Zviagina (1993) (among others), bo values increase when octahedral A1 is replaced with the larger Fe and Mg

ions. In contrast, the substitution of Si for A1 in the

tetrahedral sheet has a smaller effect on bo, although this value increases together with tetrahedral Si content (Guidotti et al . , 1989). The correlations obtained were consistent with the

TABLE 6. Values of bo (nm) determined by XRD in micas from different horizons.

Horizon Clay (<2 I~m) Coarse gravel (>8 mm)

Ap 0.9028 0.9027 AB 0.9024 0.9015 Bt 0.9022 0.9031 C1 0.9026 0.9018 C2 0.9027 0.9010

mean 0.9026 0.9020

Dioctahedral micas in soils

TABLE 7. Correlation between values of bo (nm) and atom content of K micas in the coarse gravel fraction.

117

Correlation r Significance

bo = 0.93006-0.01127(A1TM + A1 vl) -0.9686 0.01 bo = 0.89598 + 0.01973 Fm 0.9764 0.01 bo = 0.84833 + 0.01661 Si TM 0.9455 0.05

n = 5 (soil horizons).

crystallochemical findings of these earlier studies: the bo and Fm (Fe + Mg) values were positively correlated. In addition, b0 increased with tetrahedral Si, although the significance was lower than for the relat ion between bo and Fm. The correlation equations for bo and the contents of tetrahedral and octahedral A1 showed that bo was inversely related to A1 content.

S P E C I E S O F M I C A S

We determined the mineral species of micas in the gravel fraction on the basis of crystallochemical

pa rame te r s ca lcu la ted f rom mean s t ruc tura l formulae obtained with EMPA (Tables 3 and 4). Although no attempt was made to classify the micas in the clay fraction because of the low number of points recorded, we nonetheless consider our conclusions for the gravel fraction applicable to the clay fraction.

In the R3+VI-R3+IV-x diagram (R 3+Vl= octahedral

t r ivalent cat ions; R3+lV= te t rabedra l t r iva lent cations; x= layer charge) by Weaver & Pollard (1973) (Fig. 7), K micas in coarse gravel were identified as dioctahedral micas corresponding to illites (mean value of R 3+W between 0.69 and 0.81,

.e

, 8 ' i

!.-" ))

�9 o e~ e*

ee oe

0) oe e q - - K - m i c a

i t

N a - m i c a

C ~

b

FIG. 6. Selected area electron diffraction (SAED) pattern of mica in the clay fraction from horizon Bt (point 1, Table 5). Orientation parallel to c*-b*. (a) SAED micrograph; (b) schematic diagram. Split pairs of Ok/reflections

of K mica and Na mica are displayed.

118 J.M. Martfn-Garcfa et al.

� 9 ,.o

P \ 2 0 ( ' , P , \ , , .

1 .O 0.8

TETRAHEDRAL ,qz* (AI ~v)

0.6

Mu= muscovite; II1= illite; G= glauconite

1.- K-mica Ap horiz.; 2,- K-mica AB horiz,; 3,- K-mica Bt horiz,; 4.- K-mica C~ horiz.; 5.- K-mica C2 horiz.

A.- Na-K-mica Ap horiz.; B.- Na-K-mica AB horiz.; C,- Na-K-mica C1 horiz.; O.- Na-K-mica C2 horiz.

R- Na-mica total soil

FIG. 7. Micas in coarse gravel in a selected part of a Weaver & Pollard diagram (1973), showing tetrahedral R 3+, octahedral R 3+, and layer charge for clay

minera ls . (R 3+ = trivalent cations).

and always lower than 0.85, the muscovite-illite limit); Na micas were considered close to the ideal dioctahedral mica (R 3§ = 0.92, >0.85). R 3+IV and R 3+vl increased from Na to K mica, with a decrease in layer charge.

In the MR3-2R3-3R 2 (MR3= Na+K+2Ca; 2R3= (AI+Fe-1)/2; 3R2= Mg/2) diagram by Velde (1985) (Fig. 8), K micas appear as intermediate forms between illite and muscovite, with some characteristics of phengite. Layer charges vary from high (0.87, approaching 1.00) to low (0.71, close to 0.70). The composition of Na micas was close to that of the ideal mica.

On plotting the results on a Wiewiera-type diagram (Wiewiera, 1990) for the field projection" for K micas with Mg, A1 and vacant sites in octahedra (Fig. 9), the micas studied were represented as tetrahedral Si and octahedral A1. We found that the K micas were intermediate between muscovite and phengite (octahedral A1 = 1.77-1.69). Although this diagram does not show paragonites, Na mica approached the area of the ideal aluminous diocthaedral mica (octahedral A1 = 1.94).

M R 3

o?c. ,j | I

10 2o 3 R 2

II1= il l ite; Mu= muscovite; G= glauconi te; Ph= phengi te ; Ce= celadonite

1.- K-mica Ap horiz,; 2,- K-mica AB horiz.; 3.- K-mica Bt horiz.; 4.- K-mica C1 horiz.; 5.- K-mica C2 horiz.

A.- Na-K-mica Ap horiz.; B,- Na-K-mica AB horiz.; C.- Na-K-mica C1 horiz.; D.- Na-K-mica C2 horiz.

Ft* Na-mica total soil

FIG. 8. Micas in coarse gravel from a sector of a Velde diagram (1985) showing MR 3, 2R 3, and 3R 2 for clay minerals. MR3= Na+K+2Ca; 2R3= (AI+Fe-1)/2; 3R2=

Mg/2. High charge: 1.0; low charge: 0.7,

In all three diagrams, Na-K micas were at intermediate positions between the K micas and Na micas, as a result of their mixed compositions.

In the R3+VI-R3+lV-x diagram (Fig. 7), K micas in coarse gravel from the Bt-horizon were farther from the vertex for ideal dioctahedral mica. In the MR 3- 2R3-3R 2 diagram (Fig. 8) these illites showed the lowest charge, and were located nearest to phengite in the Wiewiera diagram. This confirms that these micas are the most highly evolved of all the micas analysed here; their structural formulae showed the greatest changes as a result of pedogenesis (the Bt is the most reactive horizon in soil).

C O N C L U S I O N S

In an Ultic Haploxeralf from southeastern Spain we identified micas of the dioctahedral subgroup including interlayer K micas (Na* ~< 15) belonging


All.e7 Vl.s3

Al=.oo VI.oo MUSCOVITE

Mgl.oo AIl.oo VI.oo Mg=.oo AIo.ss Vo.87 Mg2.so Vo.so

~ COPHYLLITE

L

PHENGITE!

m

Mgs.00 ~ HLOGOPITE

Al=.',s Vo.e7 Mgo.5o Al=.oo Vo.5o Mg=.oo Al~.oo EASTONITE

8 i 4

Si3.s

- - Si3

- - Sj2. s

Si=

I .- K-mica Ap horiz.; 2.- K-mica AB horiz.; 3.- K-mica Bt horiz.; 4.- K-mice CI horlz.; 5.- K-mica C2 horiz.

A.- Na-K-mlca Ap horlz.; B.- Na-K-mica AB horlz.; C.- Na-K-mlca C1 horiz.; D.- Na-K-mica C2 horiz.

P.- Na-mica total soil

FIG. 9. Micas in coarse gravel in a Wiewi6ra diagram (1990) for K micas, with Mg, A1 and vacant sites (V) in octahedra.

to the mineral species of illites and interlayer Na micas (Na*~>80) belonging to the paragonite species. Illites were related in structure and composi t ion to muscovite and phengite, and showed variable layer charges ranging from high to low (x = 0.71-0.87).

Interlayer Na-K micas ( Na* >15 and <80) were detected with other features of the structural formulae intermediate between Na and K micas. In accordance with the limbs of the muscovite- paragonite solvus, the results of mineralogical analysis with XRD, and the SAED of clay particles, it is suggested that these micas were not discrete minerals of intermediate composition, but were intergrowths of Na a6d K micas.

In crystallochemical terms, micas in the coarse gravel had developed from ideal mica, which bad lost part of its tetrahedral A1, layer charge, and interlayer cations. The process was more evident in K than in Na micas. Ultramicromorphological features confirmed this process. The most highly

modified micas (relative to ideal mica), in terms of structural formulae and crystallochemical parameters, were from the Bt horizon, where pedogenesis is more evident. Crysta l lochemical and pedogenetic evolution can be considered simulta- neous in the micas analysed.

ACKNOWLEDGMENTS

This study was supported by the 'Ministerio de Educaci6n y Ciencia de Espafia' through Project no. PB89-0459, 'La Formaci6n de Suelos Rojos en el macizo de Sierra Nevada' and Project no. PB94-0787, 'El Proceso de Herencia de micas en Suelos Rojos Mediterr~neos. Cuantificaci6n y Ecologfa'. We thank Professors E. Galen (Univ. Sevilla, Spain) and D. Fanning (Univ. Maryland, USA), and an anonymous reviewer, for their critical comments on the manuscript and valuables suggestions. We thank the Scientific Instrumentation Centre of the University of Granada, and especially Drs. M.M. Abad, M.A. Hidalgo and F. Nieto, for their technical help. Thanks are also

120 J.M. Marffn-Garcia et al.

expressed to Karen Shashok for translating the original manuscript into English.

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