View
0
Download
0
Category
Preview:
Citation preview
e c -.
i
.
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.
.
65 *
142 &
w
. ..
P
P
28 ANGLE (degrees)
P
P
Q
I I
I I -
Q
v) 0
O r
L .- E
c,
I . I
I
I
I
I I
i
0
0
Recommended