metalresearcher - 27-1-2011 at 14:19
Many oxides occurring in lava (SiO2, Al2O3, CaO, MgO) are white, so why is lava always black ?
SmashGlass - 27-1-2011 at 14:30
Simple answer. Because it is burnt!
Only kidding. Maybe?...
But you should sooo talk to a Geologist not a chemist.
spirocycle - 27-1-2011 at 14:53
there probably is a good amount of carbon in it as well.
And we all know how easy it is for a small amount of carbon to color a white mixture
Sedit - 27-1-2011 at 15:14
Even though the oxides are white in the temperatures of the volcano you will produce various silicates and what not. The main reason for it being
black however are due to Fe3O4 and Magnesium contaminates. Keep in mind that even though the oxides go into the melt a reducing enviroment where
oxygen is limited will draw the oxygen out of the glass itself forming a suspension of metal nanoparticals that refract light in various ways. Take
copper for instance, its oxide form various forms of blue and green glazes but when the oxygen is limited in the kiln it forms some of the rare red
glazes.
Almost all ceramic glazes look either grey or white before being fired but after that they become an array of viberant colors.
PS: in responce to the carbon in the volcanic glass keep in mind there is almost no chance of any being present inside of the glass itself since the
glass forms well above the temperature at which Carbon forms CO2. Its this effect of carbon that we use in ceramics to produce the reducing enviroment
needed to produce the more exotic glazes. I normally just start shoving newspapers it the kiln just as I start the cool down cycle and never cease
until 600degree C.
[Edited on 27-1-2011 by Sedit]
spirocycle - 27-1-2011 at 15:36
So the carbon sucks out the oxygen, reducing the metals?
[Edited on 27-1-2011 by spirocycle]
Sedit - 27-1-2011 at 15:39
Yup. I have even been considering flooding the kiln with N2 sometime soon to see if I can get the viberant rainbow colors that nitrides are known to
produce.
[Edited on 27-1-2011 by Sedit]
Magpie - 27-1-2011 at 15:43
Manganese dioxide and magnetite (Fe3O4) are black. I would think lava would contain some of these components.
Sedit - 27-1-2011 at 15:46
Its what the majority of it is Magpie.
barbs09 - 27-1-2011 at 15:56
Hi when most people think of lava it is probably basaltic or andesitic lava which has a high proportion of dark mafic (ferromagnesian) minerals
including amphiboles and pyroxenes (and far lesser actual black minerals including magnetite etc). However what makes lava dark (not really black) is
its fine grain size. Lava by definition is a volcanic rock and cools fast which means no time for proper crystal formation. As a result the dark
mineralshave very small grain sizes and are disseminated through the mass giving it a dark colour.
Cheers,
AB