Attached are five photos.
Photos entitled C2b17_1 and C2b17_2 are photomicrographs of same a picture that are respectively in orthogonal- and plane-polarized light. The photomicrographs show a basaltic andesite. The rock contains altered phenocrysts of pyroxene and plagioclase set in an aphanitic matrix. In the central portion of the photomicrograph there occurs a vesicle-like structure. The structure is filled with ferromagnesian mineral that has been altered to chlorite and has a narrow rim. The rim is microgranular and appears more felsic than the host rock. My question is: what the structure is? volcanic vesicular / amygdaloidal structure or not?
Photos entitled C2b17_3 and C2b17_4 are photomicrographs of same a picture that are respectively in orthogonal- and plane-polarized light. The photomicrographs also show the basaltic andesite.
In the central portion of the photomicrograph there occurs a vesicle-like structure with a narrow microgranular rim. The structure is filled with ferromagnesian mineral that has been altered to chlorite. However, some metasomatic relicts of pyroxene can be seen, suggesting that the chlorite has been altered from clinopyroxene. Within the chlorite or chloritized ferromagnesian minerals there occur clustered minerals that are radial, fibrou. Within the radial mineral cluster some metasomatic relicts of pyroxene also can be seen. My question is: what the structure is? volcanic vesicular / amygdaloidal structure or not?
The last photo was taken on the hand specimen of basaltic andesite from which the thin-sections for the photomicrographs were made. The vesicle-like structure as shown in the above photomicrographs are black-colored in the hand specimen to the naked eye.
Dear Dr. Liu,
I am much in doubt about the use of amygdule in this case. A closer look at the small-scale part of the rock, in hand-specimen gives a different morphology of the rims as observed in the large-scale images showing the thin sections. It looks like an even rim cement in some parts. By and large, it needs more information on the type of rock (e.g. pyroclastic). Please disclose more information.
Regards
H.G.Dill
Dear Prof. Dill,
Thank you very much for the suggestion. The rock is basaltic lava instead of pyroclastic. The lava consists of phenocrysts of plagioclase, clinopyroxene and olivine and microgranular to aphanitic groundmass. The groundmass includes plagioclase laths and pyroxene. The dark-colored materials, suspected of amygdule, vary in sizes. What shown in the photomicrographs are small, generally less than 2 mm along the long axis. Under the microscope they ubiquitously have a narrow margin that is 0.n to 0.0n mm in thickness. So, you may not distinguish the rim out from the hand specimen.
Regards
Liu Wei
Dear Dr. Liu,
According to the "Atlas of igneous rocks and their textures" by MacKenzie et al. (page 71, 1982) a 'vesicular texture' means, quote:"round, ovoid, or irregular holes formed by expansion of gas." In this case the cavities (vesicles) are not filled with alteration products.
An 'amygdaloidal texture' means 'quote': "former holes occupied by late-stage magmatic and/or post-magmatic minerals, such as carbonate, zeolites, quartz, chalcedony, analcite, chlorite, and/or rarely, glass or fine groundmass".
I think, your hand specimen and your thin sections clearly show an amygdaloidal texture!
Best regards,
Guenter Grundmann
It looks like amygdaloidal texture. The cavities are filled by fine fibrous aggregates of chlorite which is secondary in origin, i.e., later filling. I don't think that is has formed due to alteration of earlier pyroxene. the cavity is rimmed by fine silica, chert? in photo 1&2..
The radial fibrous aggregate found in photograph3&4 possibly could be chalcedony? or could be some zeolites, (by their appearance).
There is nothing like metasomatic pyroxene here, As by definition the amydales are filled cavities.
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