Re: ATM Marble Tool for Plate mirror?

[Anthony Stillman <atmer@flash.net>]

Bill Gillespie sent me a couple of questions which responded to.  With his
permission I'm echoing my responce to him to the list.



>
>How is marble, a calcite, harder and tougher than glass?
>

Bill,

Not all marble is. And the really soft stuff is rarely used for tile. It is
used in scupture though.  The consituents of marble in addition to calcite
include though they are not always present, graphite, pyrite, ilmenite,
dolomite, quartz, mica, chlorite, plagioclase, epidote, diopside, fassaite
pyoxene, tremolite, wollastonite, vesuvianite, forsterite olivine, talc,
brucite, serpentine, and periclase.  Also, marble is formed under
termendous pressure, 10 kilobars and higher.  One of the highest of the
metamorphs.

Calcite is a rather soft material, 3 on the Mohs hardness scale, and when
heated, calcium carbonate dissociates into lime and carbon dioxide.  Mix in
a little Natrite before heating and wa la, soda-lime glass.  In ph netural
water, soda-lime glass is far more soluble than marble.  Calcite is not
soluble in water at all unless the water is acidic and then only slightly.
Soda, of course, is water soluble.

Regarding hardness, for comparison soda-lime glass is typically about 4 on
the Mohs and Pyrex 5.5, about as hard as steel.  Hardness can be deceiving
though.  Jadeite is about as hard as steel but it is so tuff that is makes
short work of a silicon-carbide (8-8.5) wheels.

Incidently, the Mohs hardness scale is not totally linear.  Up to 9
(alumninum oxide, otherwise know as saphire) it is fairly linear.  That it,
corundum is about three times harder than calcite.  But diamond, 10 on the
Mohs is 20 to 50 times harder than saphire.  New materials like cubic boron
nitride and moissanite, in the hardness realm between ruby and diamond, are
continuing the linear trend.  Hence when you see numbers like 9.5 you know
its a bit harder than corundum and still way softer than diamond.  Even
diamond varies in hardness dependent on the crystal axis orientation.  This
is the reason diamond can be used to grind diamond.

As a sanity check I rummaged though my many boxes of rock and found two
pieces of marble, one a stark white the other pink and striated.  The first
was scratched by a piece of soda-lime and left a white streak when
scratched on the rough glass, the second scratched the soda-lime.  Perhaps
I was a little overzealous, I guess I just think of marble as being harder
and this.  It does seem to be the week for my eating crow.

Anthony