[Clayart] Crystal cleavage

[Joseph Herbert]

Hello,

A crystal structure question??

First, the quartz/glass thing: the quartz crystal structure is a three
dimensional network of silicon and oxygen atoms bonded so each silicon is
shared by four oxygen atoms. The chemical formula, SiO2, represents this
arrangement. The bonds are covalent and very strong. The material has the
same strength in all directions.

All the chemical bonds are the same strength so there are no directional
weaknesses inside the quartz crystal. No weakness, no cleavage.  Glass, on
the other hand, has no weak directions because the bonds are pretty
randomly arranged. Same result, conchoidal fracture, for opposite reasons.


The cleavage angle of plagioclase (Na-Ca) feldspars does vary with
composition but does not change drastically. In addition, plagioclase
feldspars are more chemically complex than soda or potassium feldspars. In
a general chemical formula for Na/Ca feldspars, both the sodium/calcium and
the aluminum/silicon ratios are shown as variable. This is necessary to
balance the charges in the crystal. If a sodium(+1) in a hypothetical
crystal is replaced by a calcium (+2), one silica group (-1) must be
replaced by an alumina group (-2) in the crystal.  Things are more complex
than this because the crystal lattice is continuous and at a point in the
lattice where one expects an alkali metal ion, it may be either; it is
necessary to look at the bulk composition of the crystal to determine the
ratio of ions. Probably one reason these feldspars are not used much in
pottery: wide composition variation.  This is called a double substitution
solid solution.

In sodium/potassium feldspars the charge is the same (+1) but the ion size
is different.

Cleavage in minerals is a direct result of the regular arrangement of atoms
that make up the mineral. In most minerals there are imaginary planes of
weakness where the chemical bonds across the plane are weaker than those
along the plane.  This is potentially a cleavage plane. Some separate
easily, like mica; some with difficulty, like diamond. Both of these are
called perfect cleavages, planes are very flat and smooth.

Another mineral with good cleavages is salt, halite. Three at right angles;
salt is always in cubes, big cubes , small cubes, always cubes.

Joe


“Joe,

Thanks. That is interesting, I had never thought of this. I know the
feldspars tend to be classified according to cleavage, like orthoclase, but
I've only a smattering of reading on geology. Lots of desire, but little
time.

Why doesn't quartz form cleavage moplanes? It makes sense intuitively since
glass is a direction into quartz, with pure quartz being pure glass, at
least so far as I know. But, assuming that is correct, is there another
reason?

Joel Fink.”
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