The solar system abundances of Mg, Fe, and Si dictate that chondritic meteorites are silica-deficient compared to most terrestrial or lunar igneous rocks; thus olivine-orthopyroxene assemblages are commonly observed in ordinary chondrites. However, in the unequilibrated H-chondrites Sharps, Bremervorde, and Dhajala, we have observed chondrules and fragments which contain either tridymite or cristobalite as a major phase

Brigham, Cheryl

Burnett, D. S.

Murrell, M. T.

Caltech Authors - Main

PYROXENE WHISKERS IN CP INTERPLANETARY DUST PARTICLES: EVIDENCE 
OF VAPOR PHASE GROWTH 
J.P. Bradley and D.E. Brownlee, Dept. of Astronomy, Univ. of Washington, Seattle, WA 98195 
D.R. Veblen, Dept. of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, 
MD02138 
CP micrometeorites are highly porous and fragile interplanetary dust particles (IDP' s) that 
have chondritic elemental abundances. Among the mineral species found within these particles are 
pure enstatite crystals, which are unique among the micron-sized constituents of CP IDP' s in that 
they are euhedral. They exhibit at least three distinct forms, henceforth referred to as rods, ribbons 
and platelets, whose morphologies are totally inconsistent with those of rock-forming pyroxenes 
that have crystallized from a melt. 
Chemical and structural features of the pyroxenes were investigated by analytical scanning 
and transmission electron microscopy. Rods consist of single crystals that are grossly elongated 
along the crystallographic [ 100] direction. Ribbons are blade-shaped crystals that are morphologically 
similar to rock-forming pyroxenes (Fraundorf, 1981) except that they are also elongated along 
[ 100]. Platelets consist of very thin (200-500 A) angular crystals whose habits lie in a-c projection 
(i.e. they are thin parallel to [010]). 
Each of the three morphological forms were found to possess characteristic crystallographic 
defects. For example, some of the rods contain axial screw dislocations that run parallel to the 
lengths of the crystals. High-resolution images and diffraction data indicate that they consist 
exclusively of clinoenstatite containing isolated [I 00] stacking defects. Ribbons also consist 
exclusively of clinoenstatite that contains isolated [JOO] stacking defects. Platelets consist of 
pervasively intergrown ortho and clinoenstatite, together with extreme stacking disorder. 
The presence of whiskers and platelets in CP micrometeorites has important implications with 
respect to the origin of CP IDP's. Both synthetic and naturally occurring whiskers of a variety of 
compositions have been observed (Blakely and Jackson, 1962; Veblen and Post, 1982): apart from 
their distinctive morphologies, a characteristic feature of those grown by direct vapor-to-solid 
condensation is a rod-like morphology containing an axial screw dislocation (Blakely and Jackson, 
1962). Donn and Sears ( 1963) have pointed out that nucleii present in the solar nebula are likely to 
have lead to whiskers and platelets containing screw dislocations. Such defects would have allowed 
equilibration of the saturated vapor phase with a stable crystal phase at saturations well below those 
required for other models of condensation. 
Our observations provide strong evidence that primary condensates from either the solar 
nebula or pre-solar environments have been preserved ii:t certain IDP's and that they can be 
identified and studied by electron-beam methods. Analyses of these particles should provide 
fundamental insights into the kinetic aspects of condensation. One intriguing aspect of this effort is 
the observation that growth of rods by axial screw dislocation appears to be confined to enstatite. 
Blakeley, J.M. and K.A. Jackson, 1962. J. Chem. Phys. 37, 428-430. 
Donn, B. and G.W. Sears, 1963. Science 140, 1208-1211. 
Fraundorf, P., 1981. Lunar Planet. Sci. 12th, 291-293. 
Veblen, D.R. and J.R. Post, 1982. Submitted to Am. Min. 
Si02-RICH CHONDRULES IN ORDINARY CHONDRITES 
Cheryl Brigham, M.T. Murrell and D.S. Burnett, Division of Geological and Planetary Sciences, 
Caltech, Pasadena, CA 91125 
The solar system abundances of Mg, Fe, and Si dictate that chondritic meteorites are 
silica-deficient compared to most terrestrial or lunar igneous rocks; thus olivine-orthopyroxene 
assemblages are commonly observed in ordinary chondrites. However, in the unequilibrated 
H-chondrites Sharps, Bremervorde, and Dhajala, we have observed chondrules and fragments 
which contain either tridymite or cristobalite as a major phase. Coexisting with the Si02 is 
predominantly low Ca pyroxene, as expected for a well-crystlalized, silica-saturated chondrule. 
These chondrules are striking in being conspicuously depleted in Ca and Al-rich phases: no feldspar 
was observed and only tiny grains of Ca-rich pyroxene and material of apparently glass composition 
187 
have been found. To date, we have observed approximately 20 of these chondrules/fragments; their 
abundance is 2: 3/cm2 of section. A typical object from each of the three meteorites is described 
below. 
The Bremervorde chondrule (spherical, 0.3 mm radius) is about40% Si02 (by area) with the 
Si02 concentrated towards one side, except for a few laths in the center and a discontinuous inner 
rim of Si02 grains around the pyroxene part of the chondrule. Some grains appear to be crystals 
which nucleated on the rim and grew inward. The chondrule is completely surrounded by an outer 
pyroxene rim. The interior pyroxene varies in composition, roughly radially, from En66Wo0 _4 to 
En85 W o0 . 1. In the center of the chondrule is a very small region of glass and clinopyroxene. The U 
content of the Si02 phase is 33 ppb. 
The Dhajala chondrule (spherical, 0.4 mm radius) is approximately 86% pyroxene, 8% Si02 
phase and 6% metal. The Si02 occurs as angular laths, thin concentric arcs or lamaellae oriented 
parallel to linear chains of metal blebs. Some pyroxene regions show a lamaellar (exsolution?) 
texture. Two analyses give En74Wo2 and En36Wo19 . The upper limit forthe U content of the Si02 is 
:s 8 ppb. 
The Sharps chondrule/fragment (half-circle, 0.6 mm radius) has small(~ 10 micron) Si02 
grains ( ~ 43%, volume) surrounded by interstitial layers of essentially pure fayalite. This fragment 
was completely rimmed by fine-grained olivine after being broken. Pyroxene in this object is 
somewhat more Mg-rich (Eii88Wo0 _2) than that in other Si02-bearing chondrules and appears 
uniform in FeO content. 
The bulk composition of these chondrules is probably similar to some reported by McSween 
( 1977) from unequilibrated ordinary chondrites and Kakangari, except that ours are more deficient 
in CaO and Al20 3 . Presumably, the SiO,-rich phase in the chondrules of Mcsween is typical 
chondrule glass; ours generally is crystalline. 
A simple explanation for the SiO,-rich, low Ca, Al chondrules found in Bremervorde and 
Dhajala is a process involving fractional condensation of a nebular gas. If the high temperature 
fraction (2: 1350°K) is removed after condensation, there is excess Si02 left in the gas which can 
condense as Si02 . The Si02-rich chondrules from Sharps are clearly different from those observed 
in Bremervorde and Dhajala. The texture of the Sharps objects is suggestive of alteration or 
decomposition. The fayalite-Si02 assemblages observed in Sharps could be explained by rapid 
quenching of the immiscible liquids formed in the two liquid regions of the Ft:O-Mg0-Si02 system. 
One of these liquids would yield Mg-rich pyroxene and Si02 ; the other would produce ferrosilite 
and Si02 which would decompose to form fayalite and Si02 . 
McSween, H.Y., 1977. Geochimica-Cosmochim. Acta 41, 1843-1860. 
ON THE USE OF IRON BY THE ESKIMOS OF GREENLAND 
V .F. Buchwald, Dept. of Metallurgy, Build. 204, DTH, 2800 lyngby, Denmark 
In the large archaeological material which has been collected upon Greenland and which is 
now in the National Museum, Copenhagen, there are a few implements that, surprisingly, have 
been fabricated from iron or have cutting edges of iron. A systematic examination of 70 arrowheads, 
ulos (woman-knife) and similar samples reveals that it is possible- in spite of a general very poor 
state of conservation - with modem methods, such as metallography and electronmicroprobe 
analysis, to determine the composition and origin of the specimens. 
One part is meteoritic iron with 7,5-8,5% Ni. It is slightly to heavily coldworked and has 
attained hardnesses of 275-350 HV. Some samples may have been ground or polished to final 
shape, but forging or heat treatment has not been applied. The origin is probably the Cape York iron 
meteorite, a group IIIA medium octahedrite. 
Another part is native iron with 1-4% Ni. It is ferritic with cementite inclusions, and is 
cold worked to 180-230 HV. The original pea-sized native iron minerals of Disko Island have lower 
hardnesses, 130-160 HV. 
The third part of the old tools contains wrought iron made in Denmark and Norway in the 
medieval ages; it is speculated that the Eskimos acquired it by trading with the Norsemen, 
1000-1400 A.D., and in a later period by excavating the abandoned farms of the Norsemen. 
The Eskimos are thus unique in having utilized three different types of iron for their tools. 
188 


SiO2-rich condrules in ordinary chondrites

https://authors.library.caltech.edu/57242/1/Burnett_1982p187.pdf

SiO2-rich condrules in ordinary chondrites

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