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THE STRUCTURAL RELATIONSHIP of GLAUCONITE and MICA Joun

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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 prob- able,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 photo- graphs. 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 exceedingly great 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 simi- larity 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. 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Another factor to be consideredis the preferred orienta- tion 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, ver- miculites, 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 en- tirely 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 analy- sis. It is typical for all of them. ]OURNAL MINERALOGICAL SOCIETY OF AMERICA 703 bhhbaqD N -d"o-*O<,o****dcic\ci+*ci ON iC) F 40 N 8 B€HSS€SBFRTS;PE*;$$NN oi <.-;€d6i 6i Gi6i 6i -i dJ-i-;-;-; -;J-: ai 4b q €s . 6es . i FrNFco"oNFH;:o*niOm_ a zF E a F A SAKB'$gEEREI$HEEEH€XRR F oi !.edd6i si6i -i 6i -i d-i*-iJJJ-; *-; -i 4 4Q 4Q4€4 4bQ o - - d n o ci ci n o - * E cj cj ci S ci "o o -dd 3 Fr O ..S9.o+qN+tsonoNdo€ @ 6-- n ea;€38*€;ax3=3-e3fr3 rc $ RxX o\didreooNNsoai si 6i-; -; -;-; -i-; J -i-;-; a q z D -3---- ob lj o oor*r--cj+o-i:oc;cjSd<.dd i- o N rl 4\O\ONN66N o f, :BBsiSisHgXIs$$pEEH$$ER R = g<.+.r*j ;6i 6i^i ^i6i ^i 6i _-j_; -j_:_:_; _ 6J J 4 qqq ehe bh n S N.?) OOONO ooo eoo o: tr o € D$trb$ :r qNo ns + e 5€= g3 6i mFo"io;d& € 3$* .+, s NNNNN N*J -i -iJ-; --i J F 4b4q4b N HO{TOO oNO O O N o z O 9\Or dO\N N ao\o6N€ N 4\O O\ eN N€<l +r o., € <il dl€oHO 4AN qq ..: .l .EHx +s€-j6i 6i.id "l t9h 6; .E.E; ililll is* - - e r+ e \3 N € o' H ^].a.+ b \o N I -rO € O.
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