THE STRUCTURAL RELATIONSHIP of GLAUCONITE and MICA Joun

THE STRUCTURAL RELATIONSHIP OF GLAUCONITE AND MICA

JouN W. GnuNnn, (Jniaersity oJ l[innesota, II inneapoli s, M innesota.

INrnonucrrow The composition, properties and origin of glauconite have been investigatedextensively in the past, but no attempt has beenmade to determine its structure. The writer in following up his work on other layer structures r-rayed eight glauconites under a number of different conditions and arrived at the conclusion that glauconite is a mica in structure. He is indebted to Dr. ClarenceS. Ross and Dr. W. l'. Foshag for gifts of.analyzed and unanalyzed samples of glauconite.Mr. Gilman Berg was of great assistancein purifying some of the samplesby the dielectric method. Generousgrants from the Graduate School of the University of Minnesota have made this and previousstructure studiespossible. Glauconitesused in the investigation were:

1. St. Francis County, Missouri. From the St. Joseph Lead Company mine, near Bonneterre. In the Bonneterre dolomite (upper Cambrian). Analysis, Table 3, as given by C. S. Ross (1, p. 10). Optical properties by Ross: negative, a:1597, 0:1.618, t:1.619; t-q:0.O22, 2V:20', 2E:33". The acute bisectrix X is nearly but not quite normal to the basal cleavage, which is good. The absorption is Z: Y 1X, pleochroism Z and Ii lemon yellow, X dark bluish-green. 2. Huntington, Oregon. This is an unusual glauconite of which Ross (1, p. 4) says: It Iorms large compact masses of an earthy texture. In thin section it resembles massive serpentine, with large, poorly defined, smearlike areas of birefracting material. No sharply defined crystals were observed and the cleavage is not well developed. a:1.59, "y:1.62*.005, 2V:20"40", negative. Analysis, Table 3, quoted from paper by Ross. 3. Southeastern Minnesota. Franconia formation (upper Cambrian). 4. Black Hills, South Dakota. Deadwood formation (Cambrian). 5. Mobile County, Alabama (Eocene). 6. New Brunswick area, New Jersey (Eocene). United States National Museum No. 97761. Two otJrers of unknown origin.

X-ney Dara From the beginning of this study it was consideredhighly probable,based on previousoptical data, that glauconiteis very similar

699 7OO THE AMERICAN MINERALOGIST to the Iayer structures of the micas, kaolinites, vermiculites, or chlorites.This view had beenheld by a number of mineralogistsfor years.On accountof the nature of the material the powder method of x-ray analysis provided the only means of attack. Throughout the work Fe K radialion from a modified Ksanda ion tube was used.Exposures varied from 20 to 50 hours at an averageof 7 ma' at 30 kv. The radii of the cameraswere 57.3 mm. AII powderswere mounted. with collodion on silk thread, and the rods thus formed were about 0.8 mm. in diameter. The glauconiteswere so clean when separated dielectrically that no foreign lines of quartz or other minerals could be detected in the powder spectrum photographs. As may be seen in Table 1, the interplanar distances d of the various specimens are surprisingly uniform, and the relative in- tensitiesof the lines (estimated by eye) do not vary appreciably' However, the lines lack the sharpnessfound in micas of coarser crystallization. The broadening of the lines may be partly due to the extremely small size of the crystal grains of glauconite, which for a large portion of the material must be below the optimum particles sizes for the powder method. They were so fine that no grinding of the samples was necessary beyond simple crushing' Broadeningof lines could alsobe causedby slight distortionsin the crystal lattice, modifying it very slightly from monoclinic to tri- clinic symmetry. Table 1 gives the dimensions of the unit cell of each specimenand the densitiesof two oI them as determinedwith the pycnometer'. The density of No. 1 is the averageof two determinationson about 0.3 gms. No. 6 was determined on about 2.5 gms. of material. which contained a few grains oI qtrartz and limonite. The material from Oregon, No. 2, was not suitable for density determination on account of its exceedinglygreat porosity and fineness.The other glauconiteswere not availablein large enoughpure samples' If a powder diagram of glauconite were compared with one of muscovite,no similarity probably would be noticed exceptfor the basal reflections 002 and 006, which almost coincide. Even if an iron-rich mica (biotite) pattern is used for comparison the similarity is not conspicuousuntil allowanceis made for the probable difference in the dimensions of their unit cells, and remembering 1 To fill all pore space the contents oi the pycnometer were boiled under re- duced pressure. JOURNAL MINERALOGICAL SOCIETY OF AMERICA 701

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iNe+b€ts€ooiNe+har€-o---o--- J l.'i a Iri 702 THE AMERICAN M IN ERALOGIST that weak lines of mica would be absent in glauconite on account of their diffused character, even though the two structures were the same. Another factor to be consideredis the preferred orientation of the mica lamellae parallel to the thread, which causes a much greater observed intensity for basal planes. This would not be the case with glauconite becauseeach grain of powder consti- tutes an aggregateof still smallerparticles.x

ExpnnrupNtar- Dau

There can be little doubt that next to talc the micas possessthe most stable layer structures at high temperatures.Experiments on the stability of glauconites,biotites, and other layer structures were made at 750'C. The heating was carried out in air as well as in COzover periodsof twenty hours. Only the glauconites,biotites (other micas were not tested), and the almost iron-free sheridanite (12) did not collapse. Chlorites containing considerableiron, vermiculites, montmorillonite, nontronite, and stilpnomelane broke down under these conditions. Two of the diagrams of glauconite after heating are given in Table 2. They indicate a very slight ex- pansion of the lattice in the direction normal to the cleavageand a correspondingcontraction in the direction of the b-axis. Other stability experiments were conducted in steel bombs entirely lined with gold. The glauconite was heated in solutions at temperaturesas high as 300oC.over periods as long as sevendays. Some of the experimeilts are given below, and the resultant r-ray diagramsare listed in Table 2.

1. No. 2 glauconite at 200oC. in water containing a very slight amount of NHrF. Five days. Table 2, column 3. 2. The sameat 300'C. Table 2. column 4.

* Except for one line (No. 4, Table I) the agreement would be quite excellent. This line is considerably stronger and broader than any corresponding line in biotite. It also has the appearance as if its outer edge is rather variable in position between 3.624 and 3.674. Such anomalous behavior and intensity of a line, however, are not unique for glauconite. They may be found in other extremely fine-grained crystalline substances, as for example, nacrite, vermiculite, stilpnomelane, and montmorillonite. The finer the grain the more likely they will be. The writer has no explanation to ofier for them at present. The biotite which is used for comparison in Tables 1 and 5 is from a granite at Mora, Minnesota (8). Its analysis is given in Table 3. This biotite is one of five analyzed biotites which were used for:r-ray analysis. It is typical for all of them. ]OURNAL MINERALOGICAL SOCIETY OF AMERICA 703

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3. No. 2 glauconite at 300.c. in NaHCOa (concentrated solution at room tem- perature). Nine days' Gave a new mineral not yet identified' 4. No. 2 glauconite at 300'C. in MgCl, solution (5 gms' MgCIz in 20 cc' water) to Seven days. The result was a forcing apart of the mica layers and an approach vermiculite In all experiments the air above the solutions was replaced by CO2' Ammonium fluoride was added in experimentsNo' 1 and 2 in the hope of increasingthe mobility of the ions' It may have had this effect for the lines in the diagrams of the bomb experiments seemto be slightly sharper.on the other hand, the anomalousline, No. 4, which was mentioned above, becomesrather weak or dis- appears entirely. This is also true for the specimensheated to 750"C. Further experiments dealt with the possible replacement of the K ions. If such replacementoccurred, shifts of lines might be oI value in interpreting the structure. Experirnent No' 3 proved a failure in this respect since it formed a new compound. Thallium has been mentioned by Holzner as replacing K in stilpnomelane (15),andalsoHzOinstilpnochloran(16).Thewriterrepeatedhis experiments with essentially similar results. In the case of glauconite considerableamounts of thallium seem to be taken into the structure judging by the increaseof the specificgravity of glauconite No. 6 from 2.81 to 3.02 alter boiling in thallium nitrate solution for four hours.2No appreciableintensity differenceswere noticed in the powder diagrams of such treated material (see columns 5 and 6, Table 2). Thallium has an atomic number of 81 as compared with 19 for potassium. Its diffractive power, therefore, should be more than four times that of K and when substi- tuted in the structure it should change the intensities markedly.

were replaced.It would explain why in biotite (Mora' Minnesota) the density increasedfrom 3.151 to only 3'161 after treatment with thallium nitrate solution. The particle size in the biotite is very much larger and so its surfaceis only a small fraction of that exposedin glauconite. 2 After treatment the material was washed very carefully in boiling watet several times for long periods. JOURNAL MINERALOGICALSOCIET'Y OF AMERICA 705

CuBurcer CoNsroBRarroNs Many glauconite analyseshave been published, some of them excellent,others of doubtful material. For the present discussion only three publications need be mentioned since they contain the Iatest reliable information. Hallimond (2 and 3) on the basis of twelve selectedanalyses came to the conclusionthat:

In the groups R2O3 and RO the molecular proportions are not constant and do not stand in any simple ratio to the silica and alkalis; the ordinary substitutions of alumina for ferric iron and magnesia for ferrous iron are therefore insufficient to explain the analyses. If, however, the (Fe, Mg)O and (Fe, Al)zOa are treated as mutually replaceable, considerable improvement can be brought about. He gave the formula: R2O 4(R2Or, RO).10SiO2 nHzO. C. S. Ross (1) four yearslater in a similar study in which seven- teen analyses were used, consideredglauconite an isomorphous mixture of two end memberswith the formulas: A: 2HzO KzO 2(MgO,Fe"O). 2(Fe2"'OB,AlrO3). 10SiOr*3HrO. B : 2HzO.KrO. (MgO,Fe"O). 3(Fez"'OB,AlrOs) . 1OSiO,f 3HrO. Schneider(4) a year later wrote:

The composition and variability of giauconite are more accurately expressed by the formula: (K, Na)(Fe, 1!Ig)(Fe,Al)asi6or8.3H2O. . . .

The present writer averaged the analysesof each of the three investigatorsand comparedthe results.They are remarkably similar. It would be dificult to say which is closestto the truth. They bear out the contention that glauconite is a definite species. Real difficulty is encountered when one tries to reconcile this glauconite formula with the structure of mica. The structural formula of mica per unit cell is: (OFI)8 (Mg, Fe", Fe"', Al)s rs(SirzAl+)O+0. Therefore a biotite hardly ever contains more than 40 per cent SiOz while glauconite varies between 46 and 51 per cent SiO2. The distribution of ions in a typical mica structure is shown dia- grammatically in Fig. 1. Further discussionis best restricted to a glauconiteof which we have almost complete data. Glauconite No. 1, whose chemical compositionis given in Table 3 is chosen.Its molecularproportions are found in column 1, Table 4. On the assumption that all SiOs 706 THE AMERICAN MINERALOGIST is contained in the (SiaAl)O16tetrahedral layers typical of mica the number of SiOz"molecules" in a unit cellis twelve. The number

(si, Al)4oro

(Al, Fe, Mg)c6

(si, Al)4010

Of oiioAt "61qgg Frc. 1. Diagram showing the sequence of ionic planes in muscovite. of the other oxides is of corresponding size as shown in column 2. Column 3 gives the number of ions. Adjusting the ions now on the

Tanr,n 3. ANlrvsns ol Gr-.nucont:rrs eno Brorrrn X-navoo.

Glauconite Glauconite Biotite* No. 1 No. 2 Mora, Minn.

SiOz 48.66 49.05 ,tJ.o/ AI:OS 8.46 7.96 14.56 FezO: 18.80 19.66 3.03 FeO 3.98 0.75 23.23 Mgo 3.56 l.t7 9.24 CaO 0.62 L.J+ r.l.) NazO None 0.78 0.49 KrO 8.31 618 8.06 HrO- 194 rr.79 0.23 HzO* 4.62 t.o2 TiOr llt CrsOa 0.03 F 0.16

Total With COz

No. 1. Glenn V. Brown, analyst. No. 2. E. P. Henderson, analyst + F, F. Grout, analyst, JOARNAL MINERALOGICAL SOCIDTY OF AMEITICA basis of the number of K ions present, a typical mica would have in its structure the proportional amounts shown in column 4. This would leave an excessof SiOzand HzO. Is this excessof SiOz and HzO present as submicroscopicopal or adsorbed hydrated SiOz around the mica particles? Such SiOz would not be easily detectableby c-rays.There is a seriousobjection to suchan hypoth- esis.It could not explain the relative constancy of SiOz in glauconites.The optical propertiesshould also be much more variable.

Tnsr,o 4. Mor,acur,ln ,lnl IoNrc Rnrros rN Gr,aucoNrrn No. I

I 2 3 4 5 No. of Ratio of Molecular No. of Ions in "Oxide No. of Ions Ions in Ratios Glauconite Molecules' Mica

sio: .8102 12.0 12.0Si 7.8 14.0Si Al:Oa .0830 t.22 2.4 l 2.8Al I FezOs .tt17 1.74 3 .5 Fe"' 4.0Fe"' FeO .0554 0.82 0.8Fe" 1.0 Fe" i8.2 7.8 i9.5 Mgo .0883 1.31 1.3Mg I 1.sMg I CaO .0110 0.16 0.2 Ca ) o.2Ca ) KrO .0882 1.31 2 6K 2.6 30K HzO- .r076 1.59 16HrO H:O* .2564 3 .80 7.6 0H 52

The alkali content of many micas is considerably lower than the formulas require. This is also true for glauconites.The de- ficiericy is not serious.It means that not all the K positions between the layers (Fig. 1) need to be occupied. The number of "holes" dependsprobably upon the number of (Al, Fe"') ions replacing Si, for theoreticallyfor every (Al ,F""') ion in the (Si,Al)4010 tetrahedrallayer thereshould be a K ion in the structure.Under the condition under which glauconites form, a deficiency of AlzOa (soluble in some form) is probably the rule, while the supply of SiOr in colloidal or other solution is abundant. This would favor a mica structure with a minimum of Al and maximum of Si ions in the (Si, AI)4O10portions of the layers. On the assumption that of the sixteen (Si, AI) positions in the unit cell only two were occupied by Al instead of four, the Si could be distributed over fourteen positions, and other adjustments made accordingly, as shown in column 5, Table 4. The actual formula for this particular case would be, disregardingexcess H2O: THE AMERICAN M I N ERALOGIST

(OH)r. Kr(Mg, Fe", Ca)2.7(Fe"', Al)a.s(SiuAlz)Oss-sg while the structural formula would be: (OH)r. Kr(Mg, Fe")3(Fe"', Al)e(SiuAlr)Ono. On taking the "molecules" of this actual formula and placing them into the unit cell, a theoretical density of 2.83 is obtained which is slightly less than the one found (2.855). Three HzO molecules of the analysis are disregardedin this calculation, which is justified by the dehydration curve found by C. S. Ross for this glauconite (1,p. 8). The Si ions theoretically are confined to the (Si, Al)aO16tetra- hedral layers. But there are reasons to believe that they can and do enter other portions of the structure. While they may not become ions with a covalencyof six like Fe"' and Al they probably do replace a few of these occasionally. The best known example of this kind seemsto be anauxite (9, p.87). fn kaolinite the ratio SiOz:AlzOeis usually close to 2:1, while in anauxite, which has the same structure, it may be as high as 3:1. ft is probable that anauxite,just asglauconite, forms under conditionswhere the mass action of SiOzproduced a kaolinite high in SiOz.There is, however, no avoiding the fact that the unit cell can accommodateat the most fourteen to fifteen Si ions in order to remain below a density of 3.0 (which is probably never exceededby any glauconite).With fourteen Si ions in the cell, the ratio of Si:(Al, Fe, Mg) is 12:9.5in column 5, Table 4. This is a trifle too low for the structure proposed which calls for at least 14:10. It is possible that a vacancy or "hole" exists in each unit cell (statistically speaking) which in a mica that formed under conditions of high mobility of ions does not usually exist. That such vacanciesmay exist in silicatesis well known. Due to such deficiencyin cations in glauconiteOH might easily take the place of O to balance the electrostaticcharges. A structural formula covering all theseproposed changes would be: (OH)u-to.Kr-r(Mg,Fe", Ca)1 t(Fe"' , Al, Si)a e(Sire-r+All_r)Orr-ro Cer,curarrous ol SrnucruRES While the powder spectrum photographs of glauconite and mica seem to agree within reasonablelimits it was thought best to cal- culate interplanar distancesand intensitiesfor both and compare them with the observed.The muscovite structure Cl" as found JO(TRNALMINERALOGICAL SOCIETY OF AMERICA 709 by Pauling (5) and Jackson and West (6) was used for the calculations. Its correctedcoordinates are given by Jacksonand West (7)' The formula as given by Wyckoff (14, p. 165): tmi(A2!1,)+P4 - srn"dcosd was used for intensity calculations. The value of the frequency f.actorj is explainedby Wyckoff (14,p' 177).I was divided by 5000 to obtain values of suitable size. The ionic numbers of the atoms were used for their scatteringpower F' O and OH:10, Si and Al :10. The Fe",Fe"', Al and NIg combinationswhich occupy the Al positions were assumedin such proportions to give each a scat- tering power of 20.It was assumedthat allKpositions are occupied' K:18. Any reduction in their number would materially influence the theorelical intensities as indicated in column A, Table 5 by the * and - signs for the first twenty planes. Column B gives the calculated intensities as they would be if Fe"' occupied the Al positions in the (SirAl) tetrahedral layers and all of the Al ions were in the positions between the tetrahedral layers. For this case the assumption was also made that only two-thirds of the K positions are occupied. This does not mean that all Al is actually re- placedby Fe't'in the positionsof the (SiBAI)O10tetrahedral layers' It is simply a possibility. Values somewherebetween A and B are most likely the best. The intensilies for biotite, Table 5, were calculatedas those of column,4, Table 5, except that the additional four Al positions of the structure which are vacant in muscovite were also fi.llec with ions of scatteringpower' F:20. Consideringthe nature of the formula used, the agreementbe- tween observed and calculated intensities is fair for most planes. The hht and' \kt reflections do not agree any better than for other layer structures in which this behavior was pointed out previously (lr, p. 4r7). The indices of two or three weak reflections were not calculated on account of the very large amount of labor involved in comparison to the benefitsthat could be derived. On the other hand, several dozen calculated reflections of low intensity were omitted from Table 5 on account of lack of space.The rate of decreasein observed intensities in the layer structures whose particles are 710 THE AM ERICA N MI N ERALOGIST

Tesrn 5. Tuoonlrrc,tr, aNn Ossrnvtn Ilrrelrsrrrrs ron UNrt Car,r,sor Gr,eucoNrreaxn Brorr:rr

Biotite from Mora, Minn

Theoretical Theo- Observed Intensitv retical Observed Int lnten- Int. sity

190+ 160 9.970 002 10.015 390 0 27 4.985 004 5 007 5 3- z + .)..t/ 020 4.616 3 2r+ 39 4.522 110 4.601 2l itr 56 4.484 117 4 562 I 8+ 24 4 422 02r 4.497 8 1A 4 4.341 111 4.408 1 0 2 4.241 t12 4 308 0 14+ t4 4.128 022 4.192 1 3 0 4 007 112 4.065 3 48- JI 3 878 113 3.932 6 32- 3.746 023 3.797 0+ 0 3 615 113 3.661 16 52- 48 3 484 114 3.526 52 70- 42 3.349 024 3.39+ 22 1'.' -L ,i 3.323 006 3.338 111 111 64- 62 J.ZJI 3 266 64 2- + 3 111 115 3.143 4 59- 55 2.996 025 3 026 59 40 2 886 115 2.913 l6 42- 36 2 781 116 2.807 42 0 0 2 682 026 2.705 10 1+ ZJ 2.608 200 2 -654 5 24 26 2ffi8 131 2 654 8 25 19 2.587 116 2.609 25 34 26 ? \70 202 2.624 53 78 62 2.579 131 2.624 18 2 1 2.492 008 2.503 8 2 o 2.47r 202 2.512 0 5 20 2 -471 2 5rZ 0 60 62 2 399 204 2.436 83 101 119 2.399 133 2.436 +2 1a 11 2 268 040 2 307 6 34 2.266 221 2.306 34 I 1 2 261 220 2.301 2 11 JI ? ?\r 041 2.292 .)t 27 18 2.242 222 2.28r 8 0 0 2.233 204 2 2(A 0 0 0 z z.).) IJJ 2.264 26 18 2.228 221 2.266 26 JOURNAL MINERALOGICAL SOCIETY OF AMERICA 7tr

T.lsln 5. Cowtrxuno.

Glauconite Biotite from Mora, Minn.

Theoretical Theo- Observed Intensity retical Observed Int. Inten- Int. sity

0 0 2 -2ll 042 2.249 4 19 IJ 2.170 222 2 206 6 2.146 043 2.181 6 tb 70 45 2.145 206 z.lt.) 90

133 2.t45 l.tJ 2.173 I /.) 5 7 2 064 044 2.096 0 46 r.994 0010 2 003 33 tb 20 12 1.971 206 1.994 30 40 27 | 97r r37 r.994 61 7 4 1.886 208 1 908 12 IJ 9 1.886 r37 I .908 25 0 0 | 729 208 1.746 1 0 0 t.729 139 |.746 2 11 4 1.662 0012 t.669 11 40 1.656 2010 r.672 52 78 7l 1 656 139 1.672 101

18 r.) t.621 J l.) r.647 7 13 10 t.621 315 1.647 + 13 t2 1.575 314 1.599 r.) 29 1 522 2010 1.535 38 51 +z t.522 13T1 1.535 68 50 47 r.5t2 060 1.538 60 104 9+ | 5t2 331 1.538 t2r

28 27 1.495 JJJ 1 520 0.5 +-1 24 21 1.495 .).) r 1.520 A 5 1.461 20t2 1.473 8 o o r.461 1311 1.473 12 10 t.447 .)JD t.47r 18 7 z r.447 333 1.471 13 5 6 | 424 0014 1.430 7

I I 1 |.376 .tJ/ | 397 0 2 1 1 1.376 .t.tJ r.397 0 4l 4() 1.350 20t2 .360 50 b Ind. L_J 83 93 1.350 IJ IJ .360 105

L,) 27 1.308 402 ..).)l t7 58 70 1 . JJI M 2 308 260 4l 37 1.304 400 .327 50 59 50 1.304 262 .327 75 3 8 1 ?OO 20T4 1.309 1 8 10 1.299 1313 1.309 4 THE AM ERICAN MINERALOGIST

Tleln 5. Cowrnruno.

Glauconite

Theoretical Theo- Intensity retical Observed Inten- Int.

ci f-z

o 10 |.293 .t.J9 1.311 16 9 7 | 293 337 1 311 16 I' 2l 14 | 290 404 1.3t2 27 j 0 1 | 290 262 r.312 1

18 20 r.279 402 1.301 24 1 2 28 55 | 279 264 I 301 o.J

8 10 |.252 406 r.272 12 I t4 19 1.252 264 t.272 L,) 31 26 t.246 0016 1.251 43 20 18 1.236 404 | 256 27 4 0 t.236 266 r.256 o IJ 7 r.207 20t1 1 216 18 31 20 r.207 1315 t.2t6 43 11 l.) 1.205 339 1.220 5 0 2 1.205 3311 1.220 I 0 0 I.199 408 | 218 1 1 2 t.199 266 | 218 3

Co 20 03 20.11 Z,o 9.07 9.23 AO 5.24 5.33 95'00' 950 04'

b:unusually broad line. 1zd. : indistinct. below their optimum size as in glauconite shows a noteworthy behavior. As the third index of planes increases in other words, as the angles of the planes which they make with the basal plane become relatively small-the decreasein intensity is considerably greater than expectedfrom the formula given. Therefore, "steep" planes reflect relatively more intensely. The calculated sizes of the unit cells and B angles, Table 5, of biotite and glauconite agree quite well with those observed. The mean value of B for all glau- conitesis closeto 95". JOURNAL MINERALOGICAL SOCIETV OF AMERICA /lJ

CoNclusroNs It is shownby powder spectrumphotographs that the structures of glauconite and mica are almost identical. The constants of the minerals, choosinga biotite for comparison'are:

Glauconite) Biotite (Avge. of 6 specimens) (X'Iora, Minnesota) Co 20.03A 20.11A bo 9.07 9.23 do 5 24 5.33 B angle 95"00/ 95'04', Axial ratio: .5773:1.000:2.208.5773:l.A0O:2-179

The theoretical plane 100 for a g angle of 90o as usually quoted for biotite becomes the plane 302 in this new orientation, which correspondsto that of muscovite. Glauconite is as stable as biotite at temperatures as high as 750oC.,which is additional proof of its structure. Glauconite ab- sorbs considerable amounts of thallium ions which probably replace K ions to an extent not exceeding25 per cent. The high SiOz and HzO content of glauconi'teseenxs to be d'ue to the enaironmentin which it i'sformed. Excess of soluble SiOz over available soluble AlzOs gives rise to a higher Si:Al ratio in the (Si, AI)EOrotetrahedral layers than in mica. Il may even cause substitution of a few Si ions for Fe"' or Al in approximately the positions having a covalency of 6. These Si ions would not neces- sarily be hexavalent, however. It is also thought that in comparison with a muscovite glauconite may have occasionalvacant positions or "holes" in its structure. A formula which would take into account such probabilities is: (OH)u-,o'Kr-r(Mg, Fe", Ca)r-a(Fe'/',Al, Si)ro(Sire-uAlz-a)Og8-ao' The formula for a specificcase (glauconite No. 1) is: (OH)u-r'Kr(Mg, Fe", Ca)z.z(Fe"',Al, Si)n.s(SiraAla)Ott-tn' Calcium probably would be permissible to only a fraction of a per cent in glauconite.Of the H2Ogivenin analyses'probably about hall may be adsorbed. RnrBnBNcns

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