Dear Mr. Marketing:

The principal differences controlling the rock color are found among the rock-forming minerals. It is rather simple. The basaltic igneous rocks contain pyroxene (CPx, OPx) often altered to amphibole, olivine and opaque minerals such as ilmenite and (titaniferous) magnetite and Fe-enriched glass. They are of dark mineral color. Fine-grained plagioclase which is present as well gets "drowned" in the mostly ophitic to subophitic texture. By contrast, rhyolite is a felsic volcanic rock with mainly alkaline feldspar, quartz, often with fine-grained glassy groundmass. Although, locally, accessory minerals, e.g., CPx and ore minerals may be present as well, the bright tint of the rock-forming minerals mentioned first predominate and give it a brighter rock color thna basalt. Bimodal magmatic rocks, as you mentioned in your question, is typical of extensional geodynamic regimes and characterized by the lack of volcanic rocks of intermediate composition.

Best regards

H.G.Dill

Occurrence of mafic and felsic magmatic rocks is a really common phenomenon in some tectonic settings .In some cases it represents a bimodal magmatic association. The mafic and felsic rocks can be derived from distinct mantle and crustal partial melts respectively. In an alternative scenario the melt of felsic rocks could be derived from a parental megma via fractional crystallization, magma mixing, etc. You can compare the geochemical characteristics of the rock suites in the region. However, I suggest you to consider the age of the mafic and felsic rocks before going to any further assessments.

Dear Mr. Marketing:

The principal differences controlling the rock color are found among the rock-forming minerals. It is rather simple. The basaltic igneous rocks contain pyroxene (CPx, OPx) often altered to amphibole, olivine and opaque minerals such as ilmenite and (titaniferous) magnetite and Fe-enriched glass. They are of dark mineral color. Fine-grained plagioclase which is present as well gets "drowned" in the mostly ophitic to subophitic texture. By contrast, rhyolite is a felsic volcanic rock with mainly alkaline feldspar, quartz, often with fine-grained glassy groundmass. Although, locally, accessory minerals, e.g., CPx and ore minerals may be present as well, the bright tint of the rock-forming minerals mentioned first predominate and give it a brighter rock color thna basalt. Bimodal magmatic rocks, as you mentioned in your question, is typical of extensional geodynamic regimes and characterized by the lack of volcanic rocks of intermediate composition.

Best regards

H.G.Dill

Hi. As I know It is related to tectonical place that Magma comes up. Resource is so many important. Magma with mantle resource can be basic than magma that is combined with crustel layers that included SiO2.it brings chemical composition and it's color.

Maybe you can read some papers on pumice in the Okinawa Trough. There are two types of intermediate-acidic volcanics, black and white, with similar geochemical characteristics.

Thanks Reza

Any idea how close the physical proximity and settling time of two lavas at either end of the chemical scale ( Basalt - Rhyolite ) can be ? Could they be in the same flow?

Cheers

Paul

Hi Reza

Thank you for your informative answer. The Study Area is an Island - Skomer - which is difficult to get to in rough weather. I will try and get there to collect some Andesite specimens.

Chemical composition and texture are differents in rhyolite and basalt, such as fine grained and microlithic texture for basalt.