The twinning of some minerals are relatively common in hand species,
but not observable in the thin section scale,
(at least in the volcanic rocks),
my first thought is that the structure of quartz make it this way,
but the common twinning mineral, feldspar, are also tectosilicates,
then I thought it was because quartz might be the phases that solidified last,
making it have least space to growth in the magma,
but I think this point is weak to explain this,
does anyone know something about this?
Thanks for helping in advance.
Dear Mr. Huang,
The twin lamellae of quartz, which has grown in volcanic rocks are only visible when the crystal wafer is slightly thicker (100-500) than a normal thin sections (20-30 micron). A favorable section is perpendicular to the crystallographic c-axis of the crystal. The two accompanying sketches show the twin lamellae exactly in this section. The attached photomicrograph shows in transmitted light between crossed polarizers such twins in a quartz crystal that grew in an altered basalt. The wafer of this quartz crystal is about 500 microns thick. The twin lamellae are inclined to the plane, so they appear blurry.
Best regards,
Guenter Grundmann
Dear Mues Huang,
Feldspar (plagiocse,..) typically exhibits twin in thin section because its extinction angle is highly composition dependent and the compositional variations show up as concentric bands of varying brightness in cross-polarized light.
whereas quartz or even olivine dont exhibit twin in thin section because the twin is simply less obvious and it may not affect the color or extinction.
^_^
Hi,
The reason why twins of quartz can't be observed by making use of crossed polarized light is because the orientation of the C-axis is the same for both twins; therefore the interference colour is the same!
Hope this helps.
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