catalogue of western australian meteorite collections

------- ~ -------------------------------------
CLASSIFICATION
The classification system adopted* is :-
rI-IRONS (siderites)-consist mainly of nickel~
iron.
Basis: proportion of I II-STONy-IRONS (siderolites)-consist of
nickel-iron and stony~ nickel-iron and stony material, both in sub~
material I stantial amounts.
I Ill-STONES (aerolites)-stony material is pre~
l dominant.
The last group (111)
.
is further subdivided:-
.
Ill-STONES (i) chondrites-those stony meteorites which evince chon~
dritic structure. Chondrules are rounded, ellipsoidal or angular
insets of stony or metallic material. Chondrites include botJJ.
unaltered types, in which glass (or isotropic material generalJy 'con~
sidered to be glass) is evident, and recrystallized types in which the
glass has been partly or. completely lost and the earlier formed
minerals tend to recrystallize to inequigranular crystalline aggregates,
in which the form of the chondrules may be only faintly recog~
nisable. Chondrules may have excentro-radial (fan-shaped), grated,
or r:nicro-porphyritic growth pattern. They may be monosomatic
(formed of a single crystal) or polysomatic (formed of several
crystals).
Ill-STONES (ii) achondrites-those stony meteorites which evince no
chondritic structure (this' seems to be the strict definition but some
authorities accept rare chondrules within achondrites, and there
seems to be no clearly defined line of distinction drawn between
calcium-poor achondrites (p. 16) and chondrites).
MAJOR METEORITE GROUPS
I-IRONS
These are divided into five classes on the basis of etch-patterns developed
and nickel-iron ratios.
tea) Nickel-poor ataxites (D)-nickel less than 6 %; no etch~pattern
developed; entirely composed of kamacite.
(b) Hexahedrites (H)-nickel less than 6 %; intersecting patterns of
fine lines (Neumann lines) appear on etching; almost wholly com~
ppsed of kamacite, with some schreibersite and daubreelite, commonly
present. Granular (recrystallized) hexahedrites are given the symbol
(Hb);'
(c) Octahedrites (O)-nickel content ranges from 7-14 % ; etching
reveals octahedral lam~llar patterns due to ip.tergrowths of a and
y forms of nickel-iron (kamacite and taenite) with interstitial pIessite
(a form of eutectoid).
* The system used here is adapted from that of Prior and Hey (1953) following revisions
suggested by Mason (1962).
t E. P. Henderson (verb. comm.) suggests these are only heated zones in hexahedrites" an4
that nickel-poor ataxites do not form a valid subdivision of the iron meteorites (see also
Perry 1944). . , .' ".' - .-". ,.
13