Amencan Museum

ovitates

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 7QTH STREET, NEW YORK 24, N.Y.

NUMBER 2220 JUNE 22, 1965

The Composition of the Forest City,
Tennasilm, Weston, and Geidam Meteorites

By Brian Mason! anv H. B. Wik?

THE FOREST CITY METEORITE

This meteorite fell as a shower of stones at 5.15 p.m. on May 2, 1890,
over an area | mile wide and 2 miles long, centered at latitude 43° 20’ N.,
longitude 93° 45’ W., about 11 miles northwest of the town of Forest City,
Winnebago County, Iowa. Two large stones, weighing 34 kilograms and
30 kilograms, and thousands of smaller ones ranging from 5 kilograms
down to a few grams, were recovered; the total weight was about 122
kilograms. The largest stone is in the collection of the American Museum;
it is roughly rectangular in form, some 35 cm. by 35 cm. by 16 cm. (fig.
1). The second large stone is in the University of Minnesota. Material
from the meteorite is widely distributed in collections. The Chicago
Natural History Museum has more than 700 complete stones, with a
total weight of 22 kilograms, and Yale University has about 1000 stones,
with a total weight of nearly 30 kilograms.

The Forest City meteorite was described by Kunz (1890), and there
are numerous references to it in the more recent literature. Material of
this meteorite has been extensively used in recent years for trace-element
determinations. Since it is a typical olivine-bronzite chondrite, we de-

1 United States National Museum, Smithsonian Institution, Washington, ‘D. C.; formerly
Chairman, Department of Mineralogy, the American Museum of Natural History.

2 Research Associate, Department of Mineralogy, the American Museum of Natural His-
tory.

2 AMERICAN MUSEUM NOVITATES | NO. 2220

“4

cided to make a chemical and mineralogical analysis of it, selecting for
this purpose a stone (No. 2405) from the collection of the American Mu-
seum of Natural History.

MINERALOGICAL COMPOSITION AND STRUCTURE

The principal minerals in the Forest City meteorite are olivine and
orthopyroxene. Other characteristic minerals are nickel-iron, plagioclase

nnemepoes ©

Fic. 1. The largest stone of the Forest City meteorite, together with some of the
smaller stones.

feldspar, and troilite. Minor and accessory minerals include chromite, a
phosphate mineral (apatite or merrillite, or both), ilmenite, and native
copper (in trace amounts). Notes on some of the minerals follow.

OutvinE: The refractive indices are a = 1.674, y = 1.708, indicating
a content of 19 mole per cent of the Fe2SiO4 component, according to
the determinative curve of Poldervaart (1950). By the X-ray method of
Yoder and Sahama (1957) the composition was found to be 19 mole per
cent of the FegSiO4 component. The olivine peaks on the diffractometer
chart are sharp and well defined, indicating olivine of uniform com-
position. Keil and Fredriksson (1964), by microprobe analysis, report 18.4
mole per cent Fe2SiO, in the olivine.

1965 MASON AND WIIK: METEORITES 3

Fic. 2. Photomicrograph of a thin section of the Forest City meteorite, showing
chondrules of olivine and pyroxene (gray); black is nickel-iron and troilite, white
holes in the section. The diameter of the barred chondrule in the center is 0.5 mm.

ORTHOPYROXENE: The refractive indices are a = 1.673, y = 1.683,
indicating a content of 16 mole per cent of the FeSiO3 component, ac-
cording to the determinative curve of Kuno (1954). Keil and Fredriksson
(1964) report the Fe/Fe+ Mg mole percentage of the orthopyroxene to
be 16.0. In terms of the conventional subdivision of meteoritic ortho-
pyroxene, this falls in the composition range of bronzite. The acid-in-

4 AMERICAN MUSEUM NOVITATES NO. 2220
TABLE 1
CHEMICAL COMPOSITION OF THE Forest Crry METEORITE
Cc
Fe 16.21 Fe 27.20 Si 33.46
Ni 1.65 Si 17.31 Mg 31.80
Co 0.10 Mg 14,24 Fe 26.45
FeS 5.21 S 1.90 Al 2.22
SiO, 37.06 Ni 1.65 Na 1.74
TiOg 0.15 Ca 1.24 Ca 1.70
Al2O3 2.09 Al 1.11 Ni 1.52
FeO 9.89 Na 0.73 Cr 0.38
MnO 0.28 Cr 0.37 P 0.26
MgO 23.62 Mn 0.22 Mn 0.21
CaO 1.75 P 0.15 Ti 0.10
Na2O 0.99 Co 0.10 Co 0.09
K,0 0.07 Ti 0.09 K 0.07
P2O5 0.34 K 0.06 100.00
H,O+ 0.39 H 0.04
H,O — 0.00 (O 33.59)
CreO3 0.54 100.00
100.34

A Chemical analysis expressed as nickel-iron, troilite, and oxides
B Chemical analysis expressed as elements, with oxygen added to make 100 per cent
C Chemical analysis expressed as atom percentages, with the elimination of H, O, and S

soluble concentrate of orthopyroxene and plagioclase also contains some
fine-grained material tentatively identified as clinobronzite.

PiaciocLasE: The plagioclase feldspar is fine grained and turbid, and
only a mean refractive index, 1.536, could be measured; this corresponds
to a composition about Anjo.

NickEL-Iron: Both kamacite and taenite are present.

Forest City is a markedly chondritic meteorite, the chondrules ranging
from 0.3 mm. to 2 mm. in diameter (fig. 2). The chondrules are made up
of olivine and pyroxene and show a variety of internal structure. Many
of the olivine chondrules consist of numerous idiomorphic olivine crystals
in a turbid groundmass of lower refractive index and low birefringence,
possibly plagioclase. Barred olivine chondrules, the bars consisting of this
same turbid material, are occasionally seen. The pyroxene chondrules
are usually much finer grained than the olivine chondrules and appear
to be made up of a large number of very thin plates. The nickel-iron and
the troilite seem to be confined to the groundmass between the chon-
drules.

1965 MASON AND WIIK: METEORITES 5

The density of a piece of Forest City was determined by measuring the
apparent loss of weight on suspension in carbon tetrachloride (after
evacuation under a bell jar by an oil pump to remove air) and found to

be 3.72.

CHEMICAL COMPOSITION

The chemical analysis is given in table 1, in the conventional form
expressed as metal, troilite, and oxides; in terms of the individual ele-
ments as determined by analysis, with oxygen added to bring the total
to 100; and recalculated in atom percentages with the elimination of H,
O, and 8. The conventional form of presenting meteorite analyses in-

TABLE 2
NoRMATIVE COMPOSITION OF THE Forest Ciry METEORITE

Olivine 35.3
Bronzite 25.2
Diopside 4.3
Albite 8.4
Anorthite 1.0
Orthoclase 0.4
Apatite 0.8
Chromite 0.8
Iimenite 0.3
Troilite 5.2
Nickel-iron 18.0

volves certain assumptions, for example, that all S is present as FeS, that
Fe in excess of free metal and FeS is present as ferrous iron, that all Ni
and Co are present in the metal phase. These assumptions are probably
valid for the Forest City meteorite.

The second procedure for expressing the analysis reflects more closely
the results actually obtained by the analysis. In effect, the chemical
analysis determines the amounts of the different elements, except oxygen,
no readily applicable method for this element being available. The results
obtained are then recast in the conventional form with the assumptions
outlined above.

The expression of the analysis as atom percentages after the elimina-
tion of H, O, S (and C, if present) was used by one of us (Wiik, 1956) for
comparing analyses of different types of chondrites. Such a procedure in
effect distinguishes non-volatile elements from those likely to be lost or
gained during heating in extraterrestrial environments. The figures for

6 AMERICAN MUSEUM NOVITATES NO. 2220

Forest City show that its composition is closely similar to Wiik’s group
of chondrites with 15-19 per cent of metallic iron. These are the olivine-
bronzite chondrites of Prior’s classification (1920), and they belong to
the high-iron (H) group of Urey and Craig (1953).

The normative mineral composition, expressed in weight percentages,
is given in table 2. The observed mineral composition, determined by
point-counting a polished surface, is: nickel-iron, 18.2 per cent; troilite,
5.4 per cent; chromite, 0.6 per cent; silicates, 75.8 per cent, which cor-
responds well to that calculated as the norm. The proportion of olivine
to pyroxene agrees with estimates from thin sections and X-ray diffrac-
tion patterns.

Many determinations of minor and trace elements liave been made on

material of the Forest City meteorite. The following list of published
figures (all in parts per million) is probably incomplete:
Sc, 8.9 (Bate et al., 1960); ‘Ti, 660 (Moore and Brown, 1962); V, 78 (Wiik, unpub-
lished); Cr, 2000 (Bate et a/., 1960); Mn, 2250 (Moore and Brown, 1962); Cu, 86
(Wiik, unpublished); Ge, 4.45 (Shima, 1964); Se, 8.6 (Schindewolf, 1960); Rb, 2.71
(Gast, 1962), 2.88 (Smales et al., 1964); Sr, 10.2 (Gast, 1962); Ru, 0.14 (Wiik, 1965);
Rh, 0.21 (Schindewolf and Wahlgren, 1960), 0.14 (Wiik, 1965); Pd, 0.76 (Wiik,
1965); Ag, 0.13 (Schindewolf and Wahlgren, 1960); In, 0.0010 (Schindewolf and
Wahlgren, 1960); Sn, 0.08 (Shima, 1964); Te, 0.46 (Schindewolf, 1960); Cs, 0.102
(Smales et al., 1964); Ba, 3.3 (Reed et al, 1960), 4 (Moore and Brown, 1963); Eu,
0.073 (Bate et al., 1960); Ta, 0.023 (Atkins and Smales, 1960), 0.027 (Ehmann,
1965); W, 0.18 (Amiruddin and Ehmann, 1962); Os, 0.16 (Wiik, 1965); Ir, 0.57,
0.50 (Rushbrook and Ehmann, 1962), 0.44 (Wiik, 1965); Pt, 0.98 (Wiik, 1965);
Au, 0.28 (Wiik, 1965); Pb, 0.4 (Patterson, 1955); Th, 0.2 (Patterson, 1955), 0.0387
(Bate eé al., 1959); U, 0.06 (Patterson, 1955), 0.0103 (Konig and Wanke, 1959),
0.0099 (Hamaguchi e¢ al., 1957), 0.015 (Reed et al, 1960).

THE TENNASILM METEORITE

This meteorite fell on June 28, 1872, near the village of Tennasilm in
Estonia. The circumstances of the fall and the recovery of the material
were described by Schilling (1882), who made a chemical analysis and
identified the minerals. A single stone weighing about 28 kilograms was
found some days after the observation of the fall. ‘The coordinates of this
find, which are marked on a map by Grewingk (1878), are latitude
58° 49’ N., longitude 25° 27’ E., not latitude 58° 2’ N., longitude 26° 57’
E., as given in the Prior-Hey catalogue (1953) and Krinov (1960). The
stone was broken up by gypsies, and pieces were acquired by local peas-
ants (it was believed to be a good protection against animal sickness).
However, Schilling succeeded in recovering most of the pieces, from
which he was able to reconstruct the original form and weight.

1965 MASON AND WIIK: METEORITES 7

Wiilfing (1897) was able to account for about 23 kilograms of this
meteorite, of which more than 12 kilograms were still in Schilling’s pos-
session. Small amounts are widely distributed in collections. Major por-
tions are some 4 kilograms in the Yale University collection, 3 kilograms
in the collection of the Estonian Academy of Sciences, 3.5 kilograms in
the Naturhistorisches Museum in Vienna, and 1 kilogram in the United
States National Museum, Smithsonian Institution, Washington. The
American Museum of Natural History has three pieces, with a total
weight of 179 grams; material from one of these pieces (No. 3893) was
used in this investigation.

MINERALOGICAL COMPOSITION AND STRUCTURE

The principal minerals in the Tennasilm meteorite are olivine and
pyroxene. Other characteristic minerals are nickel-iron, plagioclase, feld-
spar, and troilite. Minor and accessory minerals include chromite and a
phosphate mineral (apatite or merrillite, or both). Notes on some of the
minerals follow.

OuivinE: The refractive indices are a = 1.680, y = 1.715, indicating a
content of 23 mole per cent of the Fe2SiO4 component, according to the
determinative curve of Poldervaart (1950). This was confirmed by the
X-ray method of Yoder and Sahama (1957), which gave the same com-
position. The olivine peaks on the diffractometer chart are sharp and
well defined, indicating olivine of uniform composition.

PyRoxENE: Much of the pyroxene is orthopyroxene, although some
polysynthetically twinned clinopyroxene is also present. The refractive
indices of the orthopyroxene are a = 1.677, y = 1.688, indicating a con-
tent of 20 mole per cent of the FeSiO3 component, according to the
determinative curve of Kuno (1954). In terms of the conventional subdi-
vision of meteoritic orthopyroxene, this falls in the composition range of
hypersthene. The indices of the clinopyroxene are similar to, or a little
lower than, those of the orthopyroxene. The clinopyroxene is either clino-
hypersthene or pigeonite, the exact classification depending upon the
chemical composition.

PiaciocLase: Much of the plagioclase (isolated from an acid-insoluble
residue of the meteorite by density separation) is very fine grained and
turbid, which makes refractive index determinations difficult. On the
most suitable grains, a = 1.531, y = 1.539, corresponding to albite with
composition about Anz.

The structure of Tennasilm is markedly chondritic, the chondrules
ranging in diameter from about 0.4 mm. up to 2 mm. (fig. 3). The chon-

8 AMERICAN MUSEUM NOVITATES NO. 2220

Fic 3. Photomicrograph of a thin section of the Tennasilm meteorite, showing
chondrules of olivine and pyroxene (gray); black is nickel-iron and troilite, white
holes in the section. The diameter of the largest chondrule is 2 mm.

drules are made up of olivine and pyroxene and show a variety of internal
structure. Barred olivine chondrules are not uncommon, the bars consist-
ing of turbid material which appears to be isotropic. Chondrules consist-
ing of idiomorphic olivine crystals in this turbid groundmass are also
common. The pyroxene chondrules usually show the typical eccentric
radiating structure. The nickel-iron and troilite are virtually confined to

1965 MASON AND WIIK: METEORITES 9

TABLE 3
CHEMICAL COMPOSITION OF THE TENNASILM METEORITE

A B Cc

Fe 9.24 Fe 21.69 Si 36.52
Ni 1.13 Si 18.74 Mg 33.61
Co 0.06 Mg 14.94 Fe 21.25
FeS 6.33 Ss 2.31 Al 2.54
SiO: 40.13 Ca 1.42 Ca 1.95
TiO, 0.15 Al 1.25 Na 1.91
AleOg3 2.37 Ni 1.13 Ni 1.06
FeO 10.85 Na 0.80 Cr 0.38
MnO 0.31 Cr 0.37 P 0.31
MgO 24.77 Mn 0.24 Mn 0.24
CaO 2.00 P 0.17 Ti 0.10
Na2O 1.08 Ti 0.09 K 0.08
K2,0 0.07 K 0.06 Co 0.05
H2,0+ 0.51 H 0.06

H,O— 0.03 (O 36.67)

Cr203 0.54 100.00

99.97

A Chemical analysis expressed as nickel-iron, troilite, and oxides
B Chemical analysis expressed as elements, with oxygen added to make 100 per cent
C Chemical analysis expressed as atom percentages, with elimination of H, O, and S

the areas between the chondrules, and some of the chondrules are partly
mantled with grains of nickel-iron.

The density of this meteorite, determined as described for Forest City,
is 3.56.

CHEMICAL COMPOSITION

The chemical analysis is given in table 3, in the conventional form
expressed as metal, troilite, and oxides; in terms of the individual ele-
ments as determined by analysis, with oxygen added to bring the total to
100; and recalculated in atom percentages, with the elimination of O,
H, and S. A discussion of these procedures is given under the description
of the Forest City meteorite.

The composition of the Tennasilm meteorite shows that it is an olivine-
hypersthene chondrite in Prior’s (1920) classification. The total iron con-
tent, 21.69 per cent, is that of a low-iron (L) group chondrite of Urey and
Craig (1953). Tennasilm is noteworthy in having a somewhat lower FeO
and higher Fe content than most olivine-hypersthene chondrites.

The normative mineral composition, expressed as weight percentagets,

10 AMERICAN MUSEUM NOVITATES NO. 2220

is given in table 4. The actual mineral composition, obtained by point-
counting a polished surface, is nickel-iron, 11.1 per cent; troilite, 4.0 per
cent; chromite, 0.5 per cent; and silicates, 84.4 per cent. This is in good
agreement with the norm. The proportion of olivine to pyroxene is con-
sistent with estimates from thin sections and X-ray diffraction patterns.
No diopside was seen, but the small amount of this component is presum-
ably in solid solution in the hypersthene and clinohypersthene.

TABLE 4

NorMATIVE COMPOSITION OF THE TENNASILM METEORITE

Olivine 36.2

Hypersthene 28.7
Diopside 4,7
Albite 9.1
Anorthite 1.4
Orthoclase 0.4
Chromite 0.8
Apatite 0.9
Ilmenite 0.3
Troilite 6.3
Nickel-iron 10.4

THE WESTON METEORITE

The Weston meteorite fell near Weston, Connecticut, at 6.30 a.m. on
December 14, 1807. A shower of stones fell along a zone about 10 miles
long. ‘The largest stone weighed about 90 kilograms, and the total weight
was probably about 135 kilograms. The phenomena of the fall and the
nature of the meteorite were carefully described by Silliman and Kings-
ley of Yale College (1809). This occurrence was noteworthy; it is the
oldest recorded meteorite fall within the United States and came at a
time when the reality of such occurrences was still not generally accepted.
As Farrington (1915, p. 12) remarked: “Especial interest attaches to the
circumstances of this fall on account of the fact that the possibility of such
an occurrence was at that time scarcely believed and the general opinion
was expressed by the President of the United States, ‘Thomas Jefferson,
in the remark that it was easier to believe that Yankee professors would
lie than to believe that stones would fall from Heaven. Subsequent evi-
dence has, however, left no doubt that the Yankee professors (Profs. Silli-
man and Kingsley of Yale), as well as other historians of the fall, were
describing a real occurrence.”

Silliman and Kingsley commented on the difficulties they experi-

1965 MASON AND WIIK: METEORITES 11

Fic. 4. Photomicrograph of a thin section of the Weston meteorite, showing
chondrules of olivine and pyroxene (gray); black is nickel-iron and troilite, white
holes in the section. The chondrules are approximately 0.5 mm. in diameter.

enced in retrieving pieces of the meteorite. Most of the stones were shat-
tered on impact or broken up by their discoverers, and the pieces were
carried off by souvenir hunters. “Strongly impressed with the idea that
these stones contained gold and silver, they subjected them to all the tor-
tures of ancient alchemy, and the goldsmith’s crucible, the forge, and the
blacksmith’s anvil, were employed, in vain, to elicit riches which existed

12 AMERICAN MUSEUM NOVITATES NO. 2220

TABLE 5
CHEMICAL COMPOSITION OF THE WESTON METEORITE

A B Cc

Fe 14.87 Fe 26.90 Si 33.61
Ni 1.54 Si 17.09 Mg 31.27
Co 0.11 Mg 13.77 Fe 26.59
FeS 5.31 N) 1.94 Al 2.41
SiO: 36.59 Ni 1.54 Ca 1.75
TiOg 0.13 Ca 1.27 Na 1.69
AlsO3 2.23 Al 1.18 Ni 1.45
FeO 11.14 Na 0.71 Cr 0.38
MnO 0.32 Cr 0.36 Mn 0.25
MgO 22.84 Mn 0.25 P 0.24
CaO 1.78 P 0.14 K 0.17
Na2O 0.95 K 0.12 Co 0.10
K2O 0.14 Co 0.11 Ti 0.09
P2Os 0.32 H 0.11 100.00
H2,0 + 0.98 Ti 0.10

H,0— 0.10 (O 34,41)

CreO3 0.52 100.00

99.87

A Chemical analysis expressed as nickel-iron, troilite, and oxides
B Chemical analysis expressed as elements, with oxygen added to make 100 per cent
C Chemical analysis expressed as atom percentages, with the elimination of O, H, and S

only in the imagination. . . . Indeed, we found it very difficult to obtain a
sufficient supply of specimens of the various stones, an object which was
at length accomplished principally by importunity and purchase.” The
major part of this historic meteorite that has survived is still at Yale Uni-
versity—one stone weighing 15 kilograms, and fragments weighing 230
grams. Apparently the only other institution with as much as a kilogram
is the British Museum (Natural History), whose catalogue lists 1060
grams of this meteorite.

It is curious that little original work has been done on this meteorite
since the description of Silliman and Kingsley a century and a half ago.
On this account we decided to reanalyze the meteorite, using for the
purpose a specimen (No. 353) in the collection of the American Museum
of Natural History.

MINERALOGICAL COMPOSITION AND STRUCTURE

The minerals identified in the meteorite are olivine, orthopyroxene,
plagioclase, nickel-iron, and chromite; a phosphate mineral (apatite or

1965 MASON AND WIIK: METEORITES 13

merrillite, or both) is present in accessory amounts. Notes on the silicate
minerals follow.

OuivinE: The refractive indices are a = 1.673, y = 1.706, indicating
a content of 18 mole per cent of the Fe2zSiO4 component, according to the
determinative curve of Poldervaart (1950). By the X-ray method of
Yoder and Sahama (1957), the composition was found to be 19 mole per
cent of the Fe2Si04 component. The olivine peaks on the diffractometer
chart are sharp and well defined, indicating olivine of uniform composi-
tion.

ORTHOPYROXENE: The refractive indices are a = 1.673, y = 1.683,
indicating a content of 16 mole per cent of the FeSiO3 component, ac-
cording to the determinative curve of Kuno (1954). In terms of the con-
ventional division of meteoritic orthopyroxenes, this falls in the composi-
tion range of bronzite.

Piacioc.asE: The refractive indices are a = 1.532, y = 1.540, cor-
responding to a composition of Ang.

The structure of Weston is highly chondritic, the chondrules ranging
in diameter from 0.2 mm. to 2 mm. (fig. 4). Chondrules are made up of
olivine or pyroxene, or both, and a wide variety of structures is seen in
them. The common form of olivine chondrule consists of a closely packed
aggregate of idiomorphic olivine crystals. Pyroxene chondrules frequently
show the characteristic eccentric radiating structure of thin plates. Some-
times, however, a pyroxene chondrule is made up of an aggregate of two
or three crystals, and these occasionally show poikilitically enclosed
olivine crystals. Other pyroxene chondrules contain thin laths of olivine
in parallel orientation. The nickel-iron and troilite is virtually confined
to the groundmass between the chondrules.

The density of the Weston meteorite, determined as described under
Forest City, is 3.66.

(CCHEMICAL COMPOSITION

The chemical analysis is given in table 5, in the conventional form
expressed as metal, troilite, and oxides; in terms of the individual ele-
ments as determined by analysis, with oxygen added to bring the total
to 100; and recalculated in atom percentages with the elimination of O,
H, and S. A discussion of these procedures is given under the description
of the Forest City meteorite.

The composition of the Weston meteorite shows that it is an olivine-
bronzite chondrite in Prior’s (1920) classification. The total iron, 26.90
per cent, is that of a high-iron (H) group chondrite of Urey and Craig
(1953).

14 | AMERICAN MUSEUM NOVITATES NO. 2220

The normative mineral composition, expressed as weight percentages,
is given in table 6. The actual mineral composition, obtained by point-
counting a polished surface, is nickel-iron, 16.8 per cent; troilite, 4.2 per
cent; chromite, 0.1 per cent; and silicates, 78.9 per cent. This is consistent
with the calculated norm. Measured troilite is a little lower than the cal-
culated figure, probably because a considerable amount of the troilite is
finely dispersed and thus tends to be underestimated in the point-count-

TABLE 6
NorMATIVE COMPOSITION OF THE WESTON METEORITE

Olivine 37.3

Bronzite 23.2
Diopside 4.3
Albite 8.0
Anorthite 1.4
Orthoclase 0.8
Apatite 0.7
Chromite 0.8
Ilmenite 0.2
Troilite 53

Nickel-iron 16.5

ing. Observed chromite is much lower than calculated chromite, mainly
because most of the chromium is evidently combined in the pyroxene.:
The proportion of olivine to pyroxene is consistent with estimates from
thin sections and X-ray diffraction patterns. No diopside was seen, but
the small amount of this component is presumably in solid solution in the
orthopyroxene.

THE GEIDAM METEORITE

The Geidam meteorite was briefly described by MacLeod and Walls
(1960). Their account of the fall reads as follows: “The Geidam meteorite
fell at a village called Garau in the Zajibiri Village Unit of Geidam Dis-
trict, Bornu Province on 6th July, 1950. The stone was recovered by the
District Head of Geidam and later presented to the Geological Survey
by the Waziri of Bornu for safe keeping.

“The stone had been broken when received and is roughly tabular in
form with a thickness between 2 and 24 in. and a maximum length and
breadth of 5 in. and 24 in... . The weight is 725 gm.”

We received a small piece of this meteorite from the Geological Survey
of Nigeria and decided to make a chemical and mineralogical analysis
of it.

1965 MASON AND WIIK: METEORITES 15

Fic. 5. Photomicrograph of a thin section of the Geidam meteorite, showing
chondrules of olivine and pyroxene (gray); black is nickel-iron and troilite, white
holes in the section. The chondrules are approximately 0.5 mm. in diameter.

MINERALOGICAL COMPOSITION AND STRUCTURE

The major minerals in the meteorite are olivine, orthopyroxene,
nickel-iron, plagioclase, and troilite. Minor minerals, present in amounts
less than 1 per cent, include chromite and a phosphate mineral (apatite
or merrillite, or both). Notes on the silicate minerals follow.

OttvinE: The refractive indices are a = 1.670, y = 1.708, indicating

16 AMERICAN MUSEUM NOVITATES NO. 2220

TABLE 7
CHEMICAL COMPOSITION OF THE GEIDAM METEORITE

A B Cc

Fe 15.68 Fe 27.20 Si 32.87
Ni 1.69 Si 17.12 Mg 32.16
Co 0.13 Mg 14.50 Fe 26.28
FeS 4.63 S 1.70 Al 2.76
SiOz 36.62 Ni 1.69 Ca 1.66
TiO2 0.13 Al 1.38 Na 1.58
AlzO3 2.62 Ca 1.25 Ni 1.55
FeO 11.04 Na 0.68 Cr 0.37
MnO 0.32 Cr 0.35 P 0.24
MgO 24.04 Mn 0.25 Mn 0.24
CaO 1.75 P 0.14 Co 0.12
NazO 0.91 Co 0.13 Ti 0.09
KO 0.07 Ti 0.08 K 0.08
P.O; 0.31 K 0.06 100.00
H.O+ 0.57 H 0.06

H,O- 0.00 (O 33.41)

Cr2O3 0.51 100.00

101.02

A Chemical analysis expressed as nickel-iron, troilite, and oxides
B Chemical analysis expressed as elements, with oxygen added to make 100 per cent
C Chemical analysis expressed as atom percentages, with the elimination of O, H, and S

a content of 19 mole per cent of the FegSiO4 component, according to the
determinative curve of Poldervaart (1950). This has been confirmed by
the X-ray method of Yoder and Sahama (1957). The olivine peaks on
the diffractometer chart are sharp and well defined, indicating olivine
of uniform composition.

ORTHOPYROXENE: The indices are a = 1.673, y = 1.683, indicating a
content of 16 mole per cent of the FeSiO3 component, according to the
determinative curve of Kuno (1954). In terms of the conventional subdi-
vision of meteoritic orthopyroxene, this falls in the composition range of
bronzite. A few grains of polysynthetically twinned pyroxene, presum-
ably clinohypersthene, were observed in the acid-insoluble concentrate
of pyroxene and plagioclase.

PLaciocLasE: The plagioclase is very fine grained and turbid from in-
clusions, and only a mean refractive index of 1.536 could be measured,
indicating a composition about Anjo.

The structure of the Geidam meteorite (fig. 5) is quite chondritic,
although many of the chondrules are rather small, about 0.5 mm. in
diameter. Some excellent examples of barred olivine chondrules can be

1965 MASON AND WIIK: METEORITES 17

observed, the bars consisting of gray to black, almost opaque material.
Pyroxene chondrules with the characteristic eccentric radiating structure
are common. The nickel-iron and troilite are distributed in irregular
grains between the chondrules.

The density of this meteorite, determined as described for Forest City,
is 3.75.

CHEMICAL COMPOSITION

The chemical analysis is given in table 7, in the conventional form
expressed as metal, troilite, and oxides; in terms of the individual ele-

TABLE 8
NORMATIVE COMPOSITION OF THE GEIDAM METEORITE

Olivine 40.8
Bronzite 21.6
Diopside 3.1
Albite rey,
Anorthite 2.8
Orthoclase 0.4
Apatite 0.7
Chromite 0.8
Ilmenite 0.2
Troilite 4.6

Nickel-iron 17.5

ments as determined by analysis, with oxygen added to bring the total to
100; and recalculated in atom percentages, with the elimination of O,
H, and S. A discussion of these procedures is given under the description
of the Forest City meteorite.

The composition of the Geidam meteorite shows that it is an olivine-
bronzite chondrite in Prior’s (1920) classification. The total iron content,
27.20 per cent, is that of a chondrite of the high-iron (H) group of Urey
and Craig (1953). Geidam is noteworthy in having a somewhat higher
FeO content than most unweathered olivine-bronzite chondrites.

The normative mineral composition, expressed as weight percentages,
is given in table 8. The actual mineral composition, obtained by point-
counting a polished surface, is nickel-iron, 17.1 per cent; troilite, 4.5 per
cent; chromite, 0.2 per cent; and silicates, 78.2 per cent. This is in good
agreement with the norm. The proportion of olivine to pyroxene is con-
sistent with estimates from thin sections and X-ray diffraction patterns.
No diopside was seen, but the small amount of this component is presum-
ably in solid solution in the orthopyroxene.

18 AMERICAN MUSEUM NOVITATES NO. 2220

ACKNOWLEDGMENTS

We are indebted to the National Science Foundation for a grant
(NSF-GP-1218) toward the expenses of this investigation. We also wish
to thank Mr. G. Robert Adlington for the photomicrographs of these
meteorites, and the Geological Survey of Nigeria for the specimen of the
Geidam meteorite.

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