05 Classification of.. - Department of Earth and Planetary Sciences

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brecciated and comminuted (Figure 22(f)). On
average, they consist mostly of orthopyroxene,
but also contain minor olivine, clinopyroxene,
chromite, plagioclase, FeNi-metal, troilite, and a
silica phase (Bowman et al., 1997).
1.05.4.3 Mesosiderites
Mesosiderites are breccias composed of roughly
equal proportions of silicates and FeNi-metal plus
troilite (Figure 23). Their silicate fraction consists
of mineral and lithic clasts in a fine-grained
fragmental or igneous matrix (e.g., Floran, 1978;
Hewins, 1983; Mittlefehldt et al., 1998). There is
essentially a continuum in particle sizes between
lithic and mineral clasts and fine-grained matrix
grains, making the distinction between matrix and
clast arbitrary. The lithic clasts are largely basalts,
gabbros, and pyroxenites with minor amounts of
dunite and rarer anorthosite (Scott et al., 2001).
Mineral clasts consist of coarse-grained orthopyroxene,
olivine, and plagioclase. FeNi-metal in
mesosiderites is mostly in the form of millimeter
or submillimeter grains that are intimately mixed
with similarly sized silicate grains (Figure 23).
The mesosiderites are divided into three petrologic
classes (Hewins, 1984, 1988) based on
Classification of Meteorites
orthopyroxene content (Figure 24): the abundance
of orthopyroxene increases from A (basaltic
in composition) to B (more ultramafic) to C
(orthopyroxenite).
Mesosiderites have been further subdivided
into four subtypes based on silicate textures,
which was originally thought to reflect increasing
metamorphic equilibration of the silicates
(Powell, 1971). The lowest metamorphic grade 1
is characterized by a fine-grained fragmental
matrix; grades 2 and 3 are characterized
by recrystallized matrix, while the highest grade
4 mesosiderites are melt–matrix breccias (Powell,
1971; Floran et al., 1978). The criteria for the
mesosiderite subtypes are summarized in Table 7.
There are several problems with this textural
classification scheme: (i) it is often difficult to
distinguish between true matrix material and
fine-grained breccia or impact melt clasts;
(ii) textures are variable in some mesosiderites
(Hewins, 1984); (iii) several grade 1 mesosiderites
contain some igneous textured matrix material
(Floran, 1978); and (iv) Hewins (1984) reinterpreted
the type-3B mesosiderite plagioclase
poikilitic matrix to have an igneous texture.
Chaunskij is an unusual, highly metamorphosed,
heavily shocked, metal-rich, cordierite-bearing
mesosiderite (Petaev et al., 2000).
Figure 23 (a) The type-1A mesosiderite Mount Padbury having numerous centimeter-sized silicate and metal clasts
dispersed in a finely divided metal–silicate matrix. (b) The type-2C mesosiderite RKPA79015 having large silicatefree
metal regions. (c) The type-3A mesosiderite Emery showing a pyroxene poikiloblastic texture. (d) Polished and
etched slab of Pinnaroo, a type-4A mesosiderite, showing coarse segregation of metal and silicate and large silicate
clast (upper center) (photograph courtesy of the Smithsonian Institution).