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Chapter 10 Sepiolite
Chapter I 0 SEPIOLITE
Sepiolite is a lath-shaped magnesium-rich clay mineral with a structure similar to attapulgite. Nagy and Bradley (1955) and Brauner and Preisinger (1956) have proposed structures that differ only in detail. The oxygens which compose the base of the silica tetrahedra form continuous sheets approximately 6.5 apart; however, the apex oxygens alternately point above and below the continuous sheet. The tetrahedra which point in the same direction are joined to form two pyroxene chains linked to form an amphibole chain with an extra silica tetrahedron added at regular intervals on each side (Nagy and Bradley, 1955). Brauner and Preisinger’s structure has three pyroxene chains linked to form two continuous amphibole chains. The sheets formed by the apices of the tetrahedra are completed by hydroxylsand magnesium ions in octahedral coordination link the sheets. The one structural arrangement has nine octahedral sites and the other only eight. Both structures have channels on both sides and top and bottom of each ribbon which contains water molecules (zeolitic water). Additional water is bound to the edge of the ribbons and hydroxyls occur in the structure proper. The ideal structural formula for sepiolite based on the Nagy-Bradley model is (Sii2) ( M ~ , ) O J O ( O H ) ~ ( ~ H ~and ) ~ (Siiz)(Mgs)O~o(OH)4 ~H~O (OHz)48HzO for the Brauner-Preisinger model. Table LVI shows the chemical compositions and Table LVII the structural formulas of nine sepiolite samples (based on the Brauner-Preisinger model). Most calculated structural formulas for sepiolite indicate a minor amount (0.04 to 1.05) of A13’ and/or Fe3’ substituting for Si4 ‘(1 1.96-10.95) in the tetrahedral sheet. The magnesium-rich sepiolites have a relatively consistant composition. A1203, Fez03 and in some samples Mn2O3 are commonly present in amounts less than one percent; the MgO content ranges from 21 to 25%. Mg fills 90-100% of occupied octahedral positions. Most of the analyses indicate sufficient cations to fill approximately eight (7.74-8.14) octahedral sites. These would fill all the sites allotted by Brauner and Preisinger and leave one vacant site if the Nagy-Bradley structure is correct. The average octahedral cation charge is 15.99 with a relatively small range of values. Martin-Vivaldi and Cano-Ruis (1955) found from 50 sepiolite analyses a mean value of 53.9% Si02 with a standard error o f ? 1.9.
a
“The mean number of octahedral cations caluculated as RO (RO = moles MgO + moles Fe,O, + MnO), (the latter expressed as MgO) is 0.6 moles per 100 g, cquivalcnt to about
+ A1,03 + FeO
128
SEPIOLITE
24% MgO. From Nagy-Bradley’s (1955) structural formula, 6Si0, . MgO * 4H,O, adding four extra molecules of water to make it comparable with the analytical data, we obtain for the silica 54.2% and for the MgO 24.2%”
Their data indicate that the moles of MgO per 100 g range from approximately 0.4 t o 0.7 with a modal value near 0.6. The moles of (A& O 3 + Fez 0 + FeO + Mn) iange from approximately 0.0 to 0.15. These data also indicate that eight octahedral sites are filled and in only a few samples is substitution of Mg by trivalent cations enough to account for the filling of only seven octahedral sites. TABLE LVI Chemical conipositions of some scpiolite samples
1
SiO, 4
0
3
TiO, Fe,O, FeO Mn,O, MnO CaO MgO NiO CUO Na,O NH, H,OH,O+ Total
2
3
4
52.50 56.10 0.60 0.42
54.97 0.26
2.90 0.20 0.70 0.05
0.21
0.47 0.34 21.31 24.30
25.35
1.13
0.09
5
6
7
8
52.00 52.97 52.43 45.82 0.40 0.86 7.05 0.05 0.21 0.70 2.24 21.70 2.40 0.20 3.14 0.17 0.90 23.35 22.50 15.08 12.32
9
46.60 0.65 0.05 16.76 1 .so 0.32 0.71 15.49
10
50.40 50.80 0.66 0.02 0.73 1.85 1.5 1
20.28 16.01 9.78
0.87 12.06 10.00 9.21 9.21
9.25 10.04
99.75 99.* 100.25**
8.16 0.58 8.80 10.48 9.90 9.45
9.48 9.41
8.12 10.30
9.92 13.68 8.63 6.82
99.74 99.71 100.05
100.50
99.79 99.51
1. Caillkre (1 936b): sepiolite, Ampandrandava, Madagascar. 2. Hathaway and Saehs (1965): sepiolite, Mid-Atlantic Ridge; analysts Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis and H. Smith. 3. Maksimovic and Radukic ( 1 96 I ) : sepiolite, Goles, south Serbia. 4. Abdul-Latif and Weaver ( I 969): scpiolite, Asia Minor, Turkey. 5. Nagy and Bradley (1955): sepiolite, Little Cottonwood, Utah, U.S.A. 6. Rogers et al. (1956): aluminous sepiolite, Tintinara, South Australia. 7. CaillPre (1936b): xylotile, Sterzing, Tyrol, 8. Caillkre (1936a): xylotile, Schneeberg. 9. CaillPre (1 936a): nickeliferous sepiolite, Nouvelle Caledonie. 10. Fahey et al. (1960): loughlinite, Sweetwater Co., Wyo., U.S.A. *includes 0.1 1% K,O. 0.12% P,O,, 0.27% CO,; **includes 0.02% K,O, 0.7670 CO,.
129
SEPIOLlTF
Although the Mg-rich variety is the most common, sepiolite-type minerals afford a wide range of compositions which is mostly accommodated in the octahedral sheet. An aluminous sepiolite described by Rogers et aL(1956) has 19% of the octahedral positions filled with Al and 26% of the charge is due t o Al. Xylotile, an iron-rich sepiolite, can have up to 9% of the tetrahedral sites and 39% of the filled octahedral sites filled with Fe3+. Fe accounts for 49% of the charge. A nickeliferous sepiolite has been described by Caillere (1936b) containing 9.78% NiOz which was assigned to the octahedral sheet (Table LVII). Fahey et al. (1960) have described a variety of sepiolite which contains 8.16% NazO and only 16.01% MgO (Table LVI). Leaching experiments indicate sodium substitutes for magnesium: 2 Na for 1 Mg. Assuming all the Na is in the octahedral sheet, a value of 9.79 is obtained for octahedral cations. Preisinger (1959) has suggested that half the Na ions substitute for the Mg ions at the edge of the octahedral sheet, the other half occurs in the channels surrounded by 6 H20 molecules. Most of the cations in the octahedral positions are the large variety; Al, a smaller cation, is relatively uncommon - this in contrast to palygorskite, where A1 commonly TABLE LVII Sepiolite structural forniulas
1
Octahedral A1
I y + Fez
+
Mg
Mn Ni Na Oct. charge
2
3
4
5
6
0.01 I .37 0.06 0.47 0.03 0.04 0.02 0.37 0.44 0.13 7.14 7.22 8.14 7.96 7.42 4.90 0.53 7.74 7.81 8.14 15.95 5.71 16.28
7
8
9
10
0.32 1.80 0.29 0.29 5.33 6.97 5.53 0.06 1.03 3.65 8.01 7.97 7.18 6.96 7.48 8.00 9.79 6.07 15.96 5.90 16.61 5.76 16.00 16.25 2.69 0.04 4.20 0.03
Tetrahedral A1
Fe3+ Si Interlayer Ca Cu NH,
0.16 0.04 0.06 0.10 0.24 0.48 0.19 0.04 0.04 0.09 1.05 1.10 0.15 11.80 11.96 11.95 11.90 11.67 11.52 10.95 10.71 11.85 11.79
0.11 0.15
M€!
}:or legend, see Table XLVI
0.22
0.06
0.15 0.43 0.15
130
SEPIOLITE
TAHLI: L V I l l Obscrvcd water losses (5%) compared with those expected from ideal formulas ( A f t c r Caillhre and Hknin, 196 1 b)
1
Zeolitic water, < 250°C Bound water, 250-620°C Hydroxyls, 620- 1000°C Total
2
11.4313.6 5.7 5.6 2.4 2.4 19.53 71.6
3
11.7 5.2 2.6 19.5
4
8.1 5.4 4.1 17.6
5
11.1 5.5 2.1 19.3
1. Ampandrandava, Madagascar. lhki Chehir, Asia Minor. Salinelles, France. ldcal sepiolite (Nagy and Bradley, 1955). ldcal sepiolite (Brauner and Preisinget, 1956)
2. 3. 4. 5.
fills half the octahedral positions. Table LVIII, from Caillkre and Henin (1961b) colnpares the observed amount of water loss for a number of sepiolites with calculated values. The comparison is much better than for attapulgite. It appears that most of the exchange capacity is due to the replacement of silicon by trivalent ions, though this is not certain. The reported exchange capacity ranges between 20 and 45 mequiv./lOO g. Sepiolite is commonly associated with attapulgite, montmorillonite, dolomite and magnesite. It is reported to have formed in lacustrine environments of high basicity (Longchambon and Morgues, 1927; Millot, 1949), pluvial lakes (Parry and Reeves, 1968), in highly saline evaporitic environments (Yarzhemskii, 1949), under basic marine ccnditions (Millot, 1964), by hydrothermal alteration of serpentine (Midgley, 1959) by hydrothermal alteration in a deep marine environment (Hathaway and Sachs, 1965) and by the solution of calcite and phlogopite (Lacroix, 1941). Sedimentary sepiolite is most commonly formed along with dolomite in highly alkaline evaporatic environments. Sepiolite apparently forms under more alkaline conditions than does attapulgite and where Si and Mg concentrations are high and A1 low.
Sepiolite is a lath-shaped magnesium-rich clay mineral with a structure similar to attapulgite. Nagy and Bradley (1955) and Brauner and Preisinger (1956) have proposed structures that differ only in detail. The oxygens which compose the base of the silica tetrahedra form continuous sheets approximately 6.5 apart; however, the apex oxygens alternately point above and below the continuous sheet. The tetrahedra which point in the same direction are joined to form two pyroxene chains linked to form an amphibole chain with an extra silica tetrahedron added at regular intervals on each side (Nagy and Bradley, 1955). Brauner and Preisinger’s structure has three pyroxene chains linked to form two continuous amphibole chains. The sheets formed by the apices of the tetrahedra are completed by hydroxylsand magnesium ions in octahedral coordination link the sheets. The one structural arrangement has nine octahedral sites and the other only eight. Both structures have channels on both sides and top and bottom of each ribbon which contains water molecules (zeolitic water). Additional water is bound to the edge of the ribbons and hydroxyls occur in the structure proper. The ideal structural formula for sepiolite based on the Nagy-Bradley model is (Sii2) ( M ~ , ) O J O ( O H ) ~ ( ~ H ~and ) ~ (Siiz)(Mgs)O~o(OH)4 ~H~O (OHz)48HzO for the Brauner-Preisinger model. Table LVI shows the chemical compositions and Table LVII the structural formulas of nine sepiolite samples (based on the Brauner-Preisinger model). Most calculated structural formulas for sepiolite indicate a minor amount (0.04 to 1.05) of A13’ and/or Fe3’ substituting for Si4 ‘(1 1.96-10.95) in the tetrahedral sheet. The magnesium-rich sepiolites have a relatively consistant composition. A1203, Fez03 and in some samples Mn2O3 are commonly present in amounts less than one percent; the MgO content ranges from 21 to 25%. Mg fills 90-100% of occupied octahedral positions. Most of the analyses indicate sufficient cations to fill approximately eight (7.74-8.14) octahedral sites. These would fill all the sites allotted by Brauner and Preisinger and leave one vacant site if the Nagy-Bradley structure is correct. The average octahedral cation charge is 15.99 with a relatively small range of values. Martin-Vivaldi and Cano-Ruis (1955) found from 50 sepiolite analyses a mean value of 53.9% Si02 with a standard error o f ? 1.9.
a
“The mean number of octahedral cations caluculated as RO (RO = moles MgO + moles Fe,O, + MnO), (the latter expressed as MgO) is 0.6 moles per 100 g, cquivalcnt to about
+ A1,03 + FeO
128
SEPIOLITE
24% MgO. From Nagy-Bradley’s (1955) structural formula, 6Si0, . MgO * 4H,O, adding four extra molecules of water to make it comparable with the analytical data, we obtain for the silica 54.2% and for the MgO 24.2%”
Their data indicate that the moles of MgO per 100 g range from approximately 0.4 t o 0.7 with a modal value near 0.6. The moles of (A& O 3 + Fez 0 + FeO + Mn) iange from approximately 0.0 to 0.15. These data also indicate that eight octahedral sites are filled and in only a few samples is substitution of Mg by trivalent cations enough to account for the filling of only seven octahedral sites. TABLE LVI Chemical conipositions of some scpiolite samples
1
SiO, 4
0
3
TiO, Fe,O, FeO Mn,O, MnO CaO MgO NiO CUO Na,O NH, H,OH,O+ Total
2
3
4
52.50 56.10 0.60 0.42
54.97 0.26
2.90 0.20 0.70 0.05
0.21
0.47 0.34 21.31 24.30
25.35
1.13
0.09
5
6
7
8
52.00 52.97 52.43 45.82 0.40 0.86 7.05 0.05 0.21 0.70 2.24 21.70 2.40 0.20 3.14 0.17 0.90 23.35 22.50 15.08 12.32
9
46.60 0.65 0.05 16.76 1 .so 0.32 0.71 15.49
10
50.40 50.80 0.66 0.02 0.73 1.85 1.5 1
20.28 16.01 9.78
0.87 12.06 10.00 9.21 9.21
9.25 10.04
99.75 99.* 100.25**
8.16 0.58 8.80 10.48 9.90 9.45
9.48 9.41
8.12 10.30
9.92 13.68 8.63 6.82
99.74 99.71 100.05
100.50
99.79 99.51
1. Caillkre (1 936b): sepiolite, Ampandrandava, Madagascar. 2. Hathaway and Saehs (1965): sepiolite, Mid-Atlantic Ridge; analysts Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis and H. Smith. 3. Maksimovic and Radukic ( 1 96 I ) : sepiolite, Goles, south Serbia. 4. Abdul-Latif and Weaver ( I 969): scpiolite, Asia Minor, Turkey. 5. Nagy and Bradley (1955): sepiolite, Little Cottonwood, Utah, U.S.A. 6. Rogers et al. (1956): aluminous sepiolite, Tintinara, South Australia. 7. CaillPre (1936b): xylotile, Sterzing, Tyrol, 8. Caillkre (1936a): xylotile, Schneeberg. 9. CaillPre (1 936a): nickeliferous sepiolite, Nouvelle Caledonie. 10. Fahey et al. (1960): loughlinite, Sweetwater Co., Wyo., U.S.A. *includes 0.1 1% K,O. 0.12% P,O,, 0.27% CO,; **includes 0.02% K,O, 0.7670 CO,.
129
SEPIOLlTF
Although the Mg-rich variety is the most common, sepiolite-type minerals afford a wide range of compositions which is mostly accommodated in the octahedral sheet. An aluminous sepiolite described by Rogers et aL(1956) has 19% of the octahedral positions filled with Al and 26% of the charge is due t o Al. Xylotile, an iron-rich sepiolite, can have up to 9% of the tetrahedral sites and 39% of the filled octahedral sites filled with Fe3+. Fe accounts for 49% of the charge. A nickeliferous sepiolite has been described by Caillere (1936b) containing 9.78% NiOz which was assigned to the octahedral sheet (Table LVII). Fahey et al. (1960) have described a variety of sepiolite which contains 8.16% NazO and only 16.01% MgO (Table LVI). Leaching experiments indicate sodium substitutes for magnesium: 2 Na for 1 Mg. Assuming all the Na is in the octahedral sheet, a value of 9.79 is obtained for octahedral cations. Preisinger (1959) has suggested that half the Na ions substitute for the Mg ions at the edge of the octahedral sheet, the other half occurs in the channels surrounded by 6 H20 molecules. Most of the cations in the octahedral positions are the large variety; Al, a smaller cation, is relatively uncommon - this in contrast to palygorskite, where A1 commonly TABLE LVII Sepiolite structural forniulas
1
Octahedral A1
I y + Fez
+
Mg
Mn Ni Na Oct. charge
2
3
4
5
6
0.01 I .37 0.06 0.47 0.03 0.04 0.02 0.37 0.44 0.13 7.14 7.22 8.14 7.96 7.42 4.90 0.53 7.74 7.81 8.14 15.95 5.71 16.28
7
8
9
10
0.32 1.80 0.29 0.29 5.33 6.97 5.53 0.06 1.03 3.65 8.01 7.97 7.18 6.96 7.48 8.00 9.79 6.07 15.96 5.90 16.61 5.76 16.00 16.25 2.69 0.04 4.20 0.03
Tetrahedral A1
Fe3+ Si Interlayer Ca Cu NH,
0.16 0.04 0.06 0.10 0.24 0.48 0.19 0.04 0.04 0.09 1.05 1.10 0.15 11.80 11.96 11.95 11.90 11.67 11.52 10.95 10.71 11.85 11.79
0.11 0.15
M€!
}:or legend, see Table XLVI
0.22
0.06
0.15 0.43 0.15
130
SEPIOLITE
TAHLI: L V I l l Obscrvcd water losses (5%) compared with those expected from ideal formulas ( A f t c r Caillhre and Hknin, 196 1 b)
1
Zeolitic water, < 250°C Bound water, 250-620°C Hydroxyls, 620- 1000°C Total
2
11.4313.6 5.7 5.6 2.4 2.4 19.53 71.6
3
11.7 5.2 2.6 19.5
4
8.1 5.4 4.1 17.6
5
11.1 5.5 2.1 19.3
1. Ampandrandava, Madagascar. lhki Chehir, Asia Minor. Salinelles, France. ldcal sepiolite (Nagy and Bradley, 1955). ldcal sepiolite (Brauner and Preisinget, 1956)
2. 3. 4. 5.
fills half the octahedral positions. Table LVIII, from Caillkre and Henin (1961b) colnpares the observed amount of water loss for a number of sepiolites with calculated values. The comparison is much better than for attapulgite. It appears that most of the exchange capacity is due to the replacement of silicon by trivalent ions, though this is not certain. The reported exchange capacity ranges between 20 and 45 mequiv./lOO g. Sepiolite is commonly associated with attapulgite, montmorillonite, dolomite and magnesite. It is reported to have formed in lacustrine environments of high basicity (Longchambon and Morgues, 1927; Millot, 1949), pluvial lakes (Parry and Reeves, 1968), in highly saline evaporitic environments (Yarzhemskii, 1949), under basic marine ccnditions (Millot, 1964), by hydrothermal alteration of serpentine (Midgley, 1959) by hydrothermal alteration in a deep marine environment (Hathaway and Sachs, 1965) and by the solution of calcite and phlogopite (Lacroix, 1941). Sedimentary sepiolite is most commonly formed along with dolomite in highly alkaline evaporatic environments. Sepiolite apparently forms under more alkaline conditions than does attapulgite and where Si and Mg concentrations are high and A1 low.