XII. On the microscopical structure of meteorites

Abstract

For some time past I have endeavoured to apply to the study of meteotes the principles I have made use of in the investigation of terrestrial cks, as described in my various papers, and especially in that on the microscopical structure of crystals (Quart. Journ. Geol. Soc. 1858, vol. xiv. 453). I therein showed that the presence in crystals of “fluid-, glass-, one-, or gas-cavities” enables us to determine in a very satisfactory manner nder what conditions the crystals were formed. There are also other ethods of inquiry still requiring much investigation, and a number of experiments must be made which will occupy much time; yet, not wishing postpone the publication of certain facts, I purpose now to give a short ccount of them, to be extended and completed on a subsequent occasion. In the first place it is important to remark that the olivine of meteorites ontains most excellent “glass-cavities,” similar to those in the olivine of avas, thus proving that the material was at one time in a state of igneous usion. The olivine also contains “gas-cavities,” like those so common in olcanic minerals, thus indicating the presence of some gas or vapour (Aussun, Parnallee). To see these cavities distinctly, a carefully prepared hin section and a magnifying power of several hundreds are required. The vitreous substance found in the cavities is also met with outside and amongst the crystals, in such a manner as to show that it is the uncrystalline residue of the material in which they were formed (Mezö-Madaras, Parnallee). It is of a claret or brownish colour, and possesses the characteristic structure and optical properties of artificial glasses. Some isolated portions of meteorites have also a structure very similar to that of stony lavas, where the shape and mutual relations of the crystals to each other prove that they were formed in situ , on solidification. Possibly some entire meteorites should be considered to possess this peculiarity (Stannern, New Concord), but the evidence is by no means conclusive, and what crystallization has taken place in situ may have been a secondary result; whilst in others the constituent particles have all the characters of broken fragments (L’Aigle). This sometimes gives vise to a structure remarkably like that consolidated volcanic ashes, so much, indeed, that I have specimens whic at first sight, might readily be mistaken for sections of meteorites. It wou therefore appear that, after the material of the meteorites was melted, considerable portion was broken up into small fragments, subsequently lected together, and more or less consolidated by mechanical and chemic actions, amongst which must be classed a segregation of iron, either in th metallic state or in combination with other substances. Apparently th breaking up occurred in some cases when the melted matter had becon crystalline, but in others the forms of the particles lead me to concluc that it was broken up into detached globules whilst still melted (Mez Madaras, Parnallee). This seems to have been the origin of some of th round grains met with in meteorites ; for they occasionally still contain considerable amount of glass, and the crystals which have been formed it are arranged in groups, radiating from one or more points on the extern surface, in such a manner as to indicate that they were developed after th fragments had acquired their present spheroidal shape (Aussun, &c.). I this they differ most characteristically from the general type of concretiona globules found in terrestrial rocks, in which they radiate from the centre the only case that I know at all analogous being that of certain ooliti grains in the Kelloways rock at Scarborough, which have undergone secondary crystallization. These facts are all quite independent of th fused black crust.