Michael Banks on thu 16 sep 99
The following is an more detailed explanation of the role of fluxes, silica
and alumina on glaze and magma viscosity, for those who might be interested
in the role of NBO's, (an acronym I mentioned in a previous post):
The role of fluxes is to attack and block the bridging oxygen-silicon
or oxygen-aluminium bonds that connect silica tetrahedra and alumina
octahedra into chains. Basically SiO4 tetrahedra have two unsatisfied
(unsaturated in chemists' terms) bonds which attach to neighbouring silicon
ions in other tetrahdra. Different fluxes attack these cross-linking bonds
to varying degrees, resulting in varying temperature ranges of efficacy and
varying viscosity of the resulting melts. Generally the low-temperature
fluxes have greater affinity and blocking power to these bridging oxygen
The best example I know of is fluorine which is more electronegative than
oxygen and replaces the bridging oxygens completely, but being monovalent
does not require to be connected to anything else. Ultimately if the F
concentration is high enough all four of the oxygens attached to silicon
will be replaced, the glaze or magma will boil and SiF4 vapour will
evaporate. Potash and soda are similarly effective in taking out the
bridging oxygen bonds, but do not effect volatility to such a great extent,
forming stable, simple molecule alkali alumino-silicate complexes which are
not appreciably cross-linked with each other. Igneous petrologists now
regard melts to be composed of "quasi-crystals" of compositions depending on
the mineral in which the melt is saturated. E.G. a molten feldspathic glaze
would be regarded as composed of feldspar-like quasicrystals. These are
mono-molecular units too fine grained to be detectable with x-ray
diffraction (XRD) for example.
The so-called secondary fluxes (Ca, Mg, Ba, Zn etc) are divalent and need to
react with two bridging oxygens. They tend to be more active at higher
temperature, but can produce very fluid melts.
If the flux concentration is low relative to silica+alumina, large
conglomerate networks of silica tetrahedra and alumina octahedra clog up the
liquid because there are few non-bridging oxygen bonds (NBO's) to terminate
them. The melt is viscous.
If the flux to silica+alumina ratio is high, bridging oxygen bonds are few
and non-bridging oxygen bonds are abundant, resulting in few large network
conglomerations. The melt is fluid because the small quasicrystals can move
about with ease.
> If I understand this correctly, your are saying the
>ratio of fluxes to (silica+Al2O3) predicts glaze viscosity. Do I have
> Dave Finkelnburg, fascinated as always with the technical understanding
>you bring to ceramics!
>Subject: Geology 101
>>If pressure, volitiles, temperature and bubbles are excluded, most igneous
>>petrologists and geochemists regard the NBO/SiO2+Al2O3 ratio (NBO's are
>>non-bridging oxygen ions = fluxes) as critical in predicting magma
>>The same should be true for glazes.
Michael McDowell on thu 16 sep 99
Thank you for taking the time to share your understanding on this subject. You
have certainly given me a much clearer picture of what is going on with magma,
and perhaps a bit with glazes too...
Of course there are still these other variables you mentioned and then set
aside. Gaseous elements, including water, bubbles, crystals, etc. Got to admit
I'm still a little curious about how those factors affect magma fluidity. Can
you recommend any good igneous petrology text for the layman?
Whatcom County, WA USA
Michael Banks on fri 17 sep 99
No worries Michael!
The parameters I passed over, being dissolved volatiles, pressure, bubbles,
crystals etc, are mainly only relevant to magma viscosity's deep below the
surface. Jeff's original comments concerned the effects of viscosity on
Dissolved gases, particularly water are only soluble in magma under pressure
deep underground, but fluidise the melt to a great degree. Gas bubbles
reduce the viscosity of magma underground because they reduce friction and
gas phases lack rigidity. In other words, bubbles decrease the bulk
viscosity, by diluting a high-viscosity fluid (the silicate magma) with
inclusions of a low-viscosity fluid (the gas bubbles). Suspended crystals
impede the movement of magma by increasing internal friction, I.E.; they
increase rigidity of the melt proportionate to their percentage in the melt,
Crystals increase the bulk viscosity by clogging the melt with a proportion
of very rigid solids. Pressure largely effects viscosity, firstly because
high confining pressure increases the solubility of volatiles which have a
fluxing effect by increasing the proportion of NBO's and secondly by
squashing some elements (notably aluminium) into a higher co-ordination
numbers (from 4 to 6 in the case of Al) in the structure of the melt.
When I was last a student in the 70's, Carmichael, Turner & Verhoogen;
"Igneous Petrology" (McGraw-Hill) was the main text. The best description
I've read of the quasicrystalline model is by Burnham C. W. , Magmas &
Hydrothermal Fluids, in a book by Barnes H. L., editor "Geochemistry of
Hydrothermal Ore Deposits" 3rd edition (John Wiley & Sons).
But perhaps the best overall discussion of the parameters controlling magma
viscosity is in the book "Volcanic Successions" by Cas R.A.F & Wright J.V.,
1987 (Allen & Unwin) -section 2.5, page 23-27.
If you live close to a university, these books should be available in the
earth sciences library. The "Volcanic Successions" book is probably the
most readable for the layman and the section on viscosity is fairly
>Thank you for taking the time to share your understanding on this subject.
>have certainly given me a much clearer picture of what is going on with
>and perhaps a bit with glazes too...
>Of course there are still these other variables you mentioned and then set
>aside. Gaseous elements, including water, bubbles, crystals, etc. Got to
>I'm still a little curious about how those factors affect magma fluidity.
>you recommend any good igneous petrology text for the layman?
>Whatcom County, WA USA