Encyclopedia of the Solar System 2nd ed

Meteorites 261

have existed in the solar system because chondrites are
numerous (Table 1). Chondrites (and many achondrites)
date back to the solar system’s formation—indeed, they
provide chronometers for it (see Sections 6.4 and 6.5)—
and represent accumulated primary nebular condensate
and accretionary products. A portion of this condensate
formed from the hot nebula as millimeter-sized Ca- and
Al-rich inclusions (CAI), mineral aggregates predicted as
vapor-deposition products by thermodynamic calculations.
These CAI, found mainly in chondrites rich in carbona-
ceous (organic) material, exhibit many isotopic anomalies
and contain atoms with distinct nucleosynthetic histories.
Other inclusions (like SiC and extremely fine diamond) rep-
resent relict presolar material. Other condensates formed
at much lower temperatures. Some—perhaps even many—
CAI may be refractory residues, not condensates.
Although most chondrites contain the same minerals,
the proportions of these and their compositions differ in the
6 or so principal chondritic chemical groups. The primary
bases for chondrite classification involve proportions of iron
as metal and silicate (in which oxidized iron—expressed as
FeO—may be present), and total iron (from Fe, FeO, and
FeS) content (Fig. 7a). The last (Fig. 10) defines mete-
orites with high and low total iron (H and L, respectively)
or low total iron and low metal (LL). Numbers of H, L, and
LL chondrites are so large (Table 1) that these are called

FIGURE 10 Silicon-normalized contents of Fe as metal and in

FeS (ordinate) vs. Fe in ferromagnesian silicates (abscissa) in

various chondritic groups. (Each diagonal defines constant total

iron content.)

the ordinary chondrites. Obviously, chondrite compositions

(typically, as in Table 3, with elements apportioned by chem-

ical form) are not continuous but, rather, quantized. Table 3

lists major element ratios diagnostic of specific chondritic

TABLE 3 Average Chemical Compositions and Elemental Ratios of Carbonaceous and Ordinary

Chondrites and Eucrites

Speciesa C1 C2M C3V H L LL EUC Speciesa C1 C2M C3V H L LL EUC

SiO 2 22.69 28.97 34.00 36.60 39.72 40.60 48.56 NiO 1.33 1.71

TiO 2 0.07 0.13 0.16 0.12 0.12 0.13 0.74 CoO 0.08 0.08

Al 2 O 3 1.70 2.17 3.22 2.14 2.25 2.24 12.45 NiS 1.72

Cr 2 O 3 0.32 0.43 0.50 0.52 0.53 0.54 0.36

Fe 2 O 3 13.55 CoS 0.08

FeO 4.63 22.14 26.83 10.30 14.46 17.39 19.07 SO 3 5.63 1.59

MnO 0.21 0.25 0.19 0.31 0.34 0.35 0.45 CO 2 1.50 0.78

MgO 15.87 19.88 24.58 23.26 24.73 25.22 7.12

CaO 1.36 1.89 2.62 1.74 1.85 1.92 10.33 Total 98.86 99.82 99.84 99.99 99.99 99.92 100.07

Na 2 O 0.76 0.43 0.49 0.86 0.95 0.95 0.29 Fe 18.85 21.64 23.60 27.45 21.93 19.63 15.04

K 2 O 0.06 0.06 0.05 0.09 0.11 0.10 0.03

P 2 O 5 0.22 0.24 0.25 0.27 0.22 0.22 0.05 Ca/Al 1.08 1.18 1.10 1.11 1.12 1.16 1.12

H 2 O+ 10.80 8.73 0.15 0.32 0.37 0.51 0.30 Mg/Si 0.90 0.89 0.93 0.82 0.80 0.80 0.19

H 2 O− 6.10 1.67 0.10 0.12 0.09 0.20 0.08 Al/Si 0.085 0.085 0.107 0.066 0.064 0.062 0.29

Fe^0 0.14 0.16 15.98 7.03 2.44 0.13 Ca/Si 0.092 0.100 0.118 0.073 0.071 0.072 0.325

Ni 0.29 1.74 1.24 1.07 0.01 CaTi/Si 0.004 0.006 0.006 0.004 0.004 0.004 0.0019

Co 0.01 0.08 0.06 0.05 0.00 Fe/Si 1.78 1.60 1.48 1.60 1.18 1.03 0.66

FeS 9.08 5.76 4.05 5.43 5.76 5.79 0.14 Fe/Ni 18.12 16.15 16.85 15.84 17.73 18.64

C 2.80 1.82 0.43 0.11 0.12 0.22 0.00 Fe^0 /Ni 9.21 5.67 2.29

S (elem) 0.10 Fe^0 /Fe 0.58 0.32 0.12

aFe includes all iron in the meteorite whether existing in metal (Fe (^0) ), FeS, or in silicates as Fe 2 +(FeO) or Fe 3 +(Fe 2 O 3 ). The symbol H 2 O−indicates
loosely bound (adsorbed?) water removable by heating to 110◦C: H 2 O+indicates chemically bound .water that can be lost only above 110◦C. (Data courtesy
of Dr. E. Jarosewich, Smithsonian Institution.)