JJHerb@aol.com on wed 2 apr 97
Basalt. Aah the name brings visions of the lava erupting from Kilauea in
Hawaii, the flood basalts of the Deccan Plateau in India, and the 15,000 foot
thicknesses of basaltic layers in Oregon and Washington. Then there is the
Devil s tower and the Giant s Causeway to at that alien and Irish flavor.
Basalt - black and heavy. Smooth and dense. The extrusive equivalent of
Gabbro.
On a less poetic note, basalt is the general name given to fine grained
(volcanic, rapidly cooled) rocks that have relatively low silica content, and
relatively iron and magnesium content. Mineralogically, basalt might be
composed of a small amount of potassium feldspar, a small amount of sodium
feldspar, and approximately equal amounts of calcium feldspar, pyroxene,
amphibole and olivine. There is no free silica, in the form of quartz in a
basalt. Basalt is usually very dark or black, tending toward green rather
than red or brown. The rock is noticeably dense being more than 25% heavier
than a comparable piece of granite or sedimentary rock. Basalt also has the
unusual property of forming columnar features when a large mass is cooled
relatively slowly. The uniformity of the rock, since the crystals are all
very small, allows the formation of evenly distributed cracks throughout the
rock mass. The cracks may run in the long direction of the rock mass, as in
the Devil s tower, of across the short direction of the mass, as in the
Palisades Sill across from Manhattan, New York. The intersection of the
cracks defines relatively uniform, roughly hexagonal pieces of rock that seem
to have been carefully stacked together. In the Giant s Causeway, the upper
surface of the hexagonal pieces are most visible, so the rock gives the
impression of a floor tiled with hexagonal tiles of differing thicknesses.
For petrographers, people who perform detailed studies of rocks, the name
basalt is not sufficient to identify the range of rock compositions that fall
into the general category of basalt. They add modifiers, like olivine
basalt, to divide the rocks into more specific groups. Most often these
names are aimed at differentiating the origin of the rock, since that is the
purpose of the petrographer s study. As you can see below, the differences
in composition can be as great as the differences in the names. The first
two of these are really basalts, the last is a fine grained rock that is
closer to granite in composition and might be called "basalt" in the
marketplace just because it might have a dark color. The compositions are
given as unity formulae:
Average of 9 Ankaramites
KNa2O - 0.09
CaO - 0.42
MgO - 0.48
MnO - 0.01
Al2O3 - 0.44
SiO2 - 1.62
FexOx - 0.42
Average of 35 Olivine Basalts
KNa2O - 0.17
CaO - 0.47
MgO - 0.35
MnO - 0.01
Al2O3 - 0.65
SiO2 - 2.02
FexOx - 0.59
Average of 5 trachytes
KNa2O - 0.86
CaO - 0.09
MgO - 0.03
MnO - 0.01
Al2O3 - 1.34
SiO2 - 4.6
FexOx - 0.36
Notice, in the basalt formulas, the amount of CaO and MgO are nearly equal.
This suggests that one could use dolomite as a starting place to make Fake
Basalt. Notice also that the amount of iron in the basalt formulas is nearly
equal to the amounts of calcium and magnesium. Indeed, one could probably
make a reasonable basalt substitute with dolomite, kaolin, soda spar, flint,
and iron oxide. You should remember that the word basalt tells you
relatively little about the actual composition of the rock. A glaze recipe
that uses basalt may yield different results with different supplies of
"basalt".
A possible source of basalt composition material for glaze can be found in
the garden supply stores. The dark, rusty colored. Foamy looking, decorative
rocks are probably scoria, the foamy, gas inflated equivalent of basalt.
They could also be industrial slag, an entirely different thing. If these
are actually volcanic rocks, the powder produced by pulverizing such rocks
should be comparable to basalt.
It seems likely that the Wedgwood Basalt ware contained no basalt at all.
Joseph Herbert
JJHerb@aol.com
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