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French, p. 1

l l

IACCEBSION NUMBER) 5 e - 8

IPAQESJ i

ICATEQORY)

Graphitization oI" Organic Bterial

In a Progressively Metamorphosed

Precambrian Iron F o m t i o n

ff 653 July 65

. .

https://ntrs.nasa.gov/search.jsp?R=19650019863 2020-06-29T15:16:27+00:00Z

French, p. 2

ABSTRACT

Organic matter in the sedimentary Biwabik iron formation in

northern Minnesota shows a progressive increaGe in crystallinity

where the Tonnation is metamorphosed by the intrusive Duluth

gabbro complex. X-ray diffraction. studies of acid-insoluble

residues

material

graphite

\

show a com2lete range in crystallinity from amorphcus

in the unmetesorphosed sediments to completely crystalline

adjacent to the 'geb'oro.

i

..

. French, p. 3

Exeninatiox of cez3onace~~s m'cerial fro= wetmor2hosed ?re-

caabrian sedkents (1) ?AS shown thct such Ete;-iaI. is not c-rysLdlioe

grzphite, It is insliead nn amorpb.ous aL&-egzte of bqdrocarbon

compounds of high molecular weight which may be related t o the so-

c&lled "kerogen" found' in y o i q e r sedicentary rocks (2) .

During a study of progressive metamorphism of the Biwabik i r o n

formt ion i n northern Minnesota (3) , the carbonacecus matter from

organic-rich layers i n the fornation was examined by X-ray diffrac-

t ion t o determine the degree of c rys ta l l in i ty a t various aetamorphic

leve ls and t o detect, i f possible, t.he developent of crystall ine

graphite during metamorphism. -

The Biwabik iron formation, on th,e Mesabi Range i n northern

Y5miesota, i s a chemical sediment of Middle Precambrian age. It

i s composed chiefly of quartz, W n e t i t e , hemt i te , s ider i te ,

ankerite, ar,d s eve rd hydrous iron s i l i c a t e s (3,4).

On the eastern ead of t k b s e b i Rage, ne= the town of Babbitt,

the seduents were intruded by t'ne Ddu'ch gabbro complex about 1.0

b.y. ago.

occur within a Zew miles of the contact (3,4,5).

mtaxorphosed iron formtion, 2 t o 3 miles ( 3 t o 5 kiloneters) from

Progressive changes i n t'ie nineralogy of the iron formt ion

The acderately

the gabbro is characterized by the presence of the iron-amphibole

1 . . . I .

French, p. 4

grunerite; ~ r u a e r i t e - c ~ ! r e r i t e - c ~ c i t e assemblages are t-y-picel. The

highly rnetamorphoszi: iron formation adjacent t o the gz%ro is char-

acterized by iron-rich pyroxenes, by reduction of hematite t o

magnetite, and by the absence of all carbonates except calci te .

The samples studied were collected along the s t r ike of the *

Biwabik formation fYom the so-called "Intermediate Slate," a

well-known and easi ly recognized marker bed.

gray t o black, fine-grained and f inely laminated layer which i s

generally 5 t o 40 f e e t (2 t o 15 meters) thick.

un i t have shown from 1 t o 4 weight per cent carbon (6).

The uni t i s a dark

Analyses of t h i s

Unmetamor-

phosed "Intermediate Slate" i s composed chiefly of s ider i te and

iron-rich chamosite, wizh minor quartz, stilpnomelane, and magnetite.

Grunerite, iron pyroxenes, and fayal i te appear where the uni t is

metamorphosed . The method of sample preparation was suggested by T. C . Hoering,

of the Geophysical Laboratory of the Carnegie Inst i tut ion of

Washington, and has been used by h i m t o separate organic components

from sedimentsry rocks.

smple were ground t o minus 230 mesh and then dissolved i n hot

hydrofluoric acid for about 24 hours t o remove s i l i ca t e and carbonate

minerals,

(101) peak of quwtz w i l l completely mask the graphite (002)

About 100 grams of the "Intermediate Slate"

It w a s essential that all quartz be removed, since the strong

' .

* ' French, p. 5

ref lect ion if even 8 small mount of quartz is present. The insolu-

ble residue w a s then treated w i t h hot hydrochloric zcid for about

24 hours t o remove fluorides and f luosi l icates . X-ray diffraction '

patterns were made t o monitor the process a t each stage.

The carbon concentrate obtained consisted of from one t o 4

grams of a dark way or black material. Small portions were

mounted on a glass s l ide i n an acetone-Duco cement mixture md

studied by X-ray diffraction, using Ni-filtered Cu K radiation.

Small amounts of original pyrite and tourmaline also survived

treatment and were identified i n the pattern..

The X-ray diffraction patterns obtained can be divided in to

four major groups which represent different degrees of c rys ta l l in i ty

' i n the organic matter (Fig. 1) :

(1) An amorphous pattern without peaks (Fig. 1, A ) , which

indicates completely noncrystalline material.

obtained only from material separated from the unmetasorphosed

iron formation.

Such patterns were

(2) A very broad peak a t a 28 value of about 25 degrees, cor-

responding t o a doo2 value of about 3.5 A (Fig. 1, B, C) . patterns resenble those of coals and asphaltic substances (7).

Such

One specimen, which produced th i s pattern, contained patches of

French, p. 6

black, b r i t t l e , resinous hydrocarbon in carbonate-bearing iron

formation.

extractible organic compunds (8).

This material contained a measurable amount of pyridine-

I (3) A diffuse peak a t a 20 value of 26.0 degrees (am2 - 3.43 A), suggestive of poorly-crystalline graphite (Fig. 1, D) . Such a pattern may represent graphite with considerable disorder

i n layer stacking ("turbostratic") (9, p. 79).

graphite" ( for "disordered") is used here t o designate such

material.

The term "d-

(4) A sharp peak at a 28 value of 26.5 degrees (aoo2 = 3.36 A) ,

(Fig. 1, 3), corresponding t o the (002) re f lec t ion of well-

crystal l ized graphite ( 9 ~ 0 ) .

Figure 2 shows the relationship between crys ta l l in i ty of

organic matter and the metamorphic grade as indicated by index

minerals i n the iron formation. A cer tain amount of "overlap" of

the various types r e su l t s from the steep thermal gradient in the

contact zone and from the fact tha t the mineralogical zones are

not exactly parallel t o the surface con"act between the i ron

formation and the gabbro (3) . Nevertheless, the r e su l t s indicate

a general increase i n crystal l ini ty of organic material with

increasing metamorphism. Organic matter from the unmetamrphosed

* i French, p. 7

iron formation is amorphous, while both graphite and d-graphite

appear t o be res t r ic ted t o the neighborhood of the gabbro. A

similar increase i n crystal l ini ty with increasing metamorphic

grade has been observed i n regionally metamorphosed coals (ll) . # The present study describes a simple method for the identi-

f icat ion of crystal l ine graphite i n rocks and for the dis t inct ion

between g a p h i t e and amorphous organic matter.

action of graphite as a "buffer" t o control oxygen fugacity (3,12)

makes the exact identification of such material i n igneous and

The possible

metanorphic rocks of particular significance. It is probable

that the presence of amarphous organic material in sediments is

significant i n fixing low values of PO2 a t intermediate leve ls of

metamorphism, although calculations based on crystall ine graphite

=e not rigorously applicable t o such material.

Department of Geology

The Johns Hopkins University

Baltimore 18, Maryland '

Bevan M. French

1.

2.

' 3.

4.

5 -

.6.

F. M. Swain, A. Blumentals,

Petrol. Geologists, 42, 173

French, p. 8

AND NOTES

N. Frokogovich. B u l l . - - - Am. Assoc.'

(1958); T. C. Hoering, Carnegie

Inst . Wash. Year Book 61 184 (1962). - - --,A

Forsman, J. P., in Organic Geochemistry, I. A. Breger, Ed.

(Pergamon Press, New York, 1963),

B. M. French. Ph.D. dissertation,

Baltimore, Maryland (1964).

pp. 148-182.

Johns Hopkins University, ~

J. W. Gruner, The - Mineralogy - and Geology -- of the Taconites - and

Iron Ores of the Mesabi R;nge, Minnesota (Office of the Com-

missioner of the Iron Range Resources and Rehabilitation,

S t . Paul, Minnesota, 1946), 12'7 p.; D. A. White, M i a . -- Geol.

-- S u r v . B u l l . 38, 92 p. (1954).

7 - v -

J. N. Gundersen and G. M. Scbar tz , Minn. Geol. Surv. B u l l . ---- 431 139 P* (1962).

Gruner, s. e. (l946), pp. 54-62.

c - French, p. 9

7 . D. W . Va? Xrevelen, Coal: Typology, Chemistry, Physics, - Constitution (Elsevier, New York, 1961), pg. 324-342; J. F.

Weiler, i n Chemist% of Coal Utilization:

H. H . Lowry, ed. (Wiley, New York, 1963), pp. 580-628.

Supplementesy Volume, --

8. T . C . Hoering, personal communication, 1962.

9. A. R . Ubbelohde and F. A. Lewis, Graphite and Its Crystal

Compomds (Clarendon Press, Oxford, 1960), 217 p.

--

10. L. G. Berry and R . M. Thompson, Geol. S O ~ . Am. Mem. €&, 23 - - - - (1962) ’

ll. A. W . @inn and H. D. Glass, Econ. Geol. 53, 563 (1958). --

12. E. Zen, &. J. Sci 261, 929 (1963); A. Miyashiro, Geochim.

e t Cosmochim. A c t a , 28, 717; B. M. French and H. P. Eugster,

- - -‘I - i n preparation.

e

- ” . . . - _ .

- .

FYench, p. 10

13. This research w a s part of a doctoral disser ta t ion completed under

the supervision of Prof. H. P. Eugster, Departmant of Geology,

The Johns Hopkins University, Baltimore, Maryland; it w a s sup-

ported i n part by a grant from the Penrose Fund of the Geological

Society of America. I thank D r . T. C. Hoering, of the Geo-

physical Laboratory of the Carnegie Inst i tut ion of Washington,

f o r many helpful suggestions on sample preparation end inter-

pretation of the X-ray diffraction patterns.

benefited greatly from discussions with D r s . H. J. Greenwood,

Jr. and D'. R. Wones.

I have also

14. The writer is presently with the Theoretical Division, Goddard

Space Flight Center, National Aeronautics and Space Administration,

Greenbelt, Maryland, as a National Acadelqy of Sciences--NBtional

Research Council. Research Associate.

French, p. 11

FIGURE CAFTIONS

F i g . 1.

samples of the "Intermediate Slate" member of the Biwabik iron

formation.

traces; the peaks designated "P" in patterns A, B, and C are

produced by small amounts of pyrite.

indicated by the gradual development of the strong (002) graphite

ref lect ion a t about 26 degrees.

material fYom unmetamorphosed iron formation.

show the gradual appearance of a broad peak indicative of asphaltic

X-ray diffract ion patterns of organic matter extracted from

Sample numbers are shown a t the left-hand side of the

'

Increasing graphitization is

Pattern A represents amorphous

Patterns B and C

substances. Pattern D is a disordered graphite, and Pattern E

represents crystal l ine graphite.

E has been reduced by one-half re la t ive t o the other patterns.

The height of the peak i n Pattern

- .-.-

F i g . 2. Crystall inity of organic matter i n the "Intermediate Slate"

member as a function of distance from the Duluth gabbro complex.

The metamorphic grade of the Biwabik iron formation is indicated

by the index minerals (calci te , grunerite, and iron pyroxenes)

which have developed from original minerals during metamorphism.

Graphite and d-graphite appear res t r ic ted t o the highly metamorphosed

iron formation.

.

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P

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