[Rockhounds] Re: Rockhounds Digest, Vol 3, Issue 21

[Rock Currier]

Dear Paul,
    Probably the best acid to use when "cleaning" up calcite is hydrocloric
acid. This is standard muriatic acid that you can buy in almost any hardware
store and is commonly used to adjust the acidity of swimming pools. Tons of
various colored calcite chunks of calcite of various colors, especially from
Mexico are routinely diped in hydrochloric acid to "acid polish" the pieces.
The common onyx carvings produced in Mexico and seen abundantly in rock
shops abundantly during the last 30 or more years is all finished with an
acid polish. If you are looking to touch up mineral specimens you should
consider diluting the concentrated pool acid with about five parts water to
one part acid. A small ammount of this acid will go a long way in touching
up specimens. Use rubber gloves and eye protection when working with acids.
The concentrated hydrochloric acid emits strong fumes and you should work
with it in a well ventilated lace and make sure the wind is blowing away
from you. Once the acid is diluted you won't have any problems with the acid
fumes. Keep a hose with running watter handy to dilute any spilled acid. You
can use bicarbonate of soda to neutralize any spilled acid. One table spoon
full of acid and five of water will be enough for most specimens. Just use a
cotton tiped Q tip, stick it in the acit and rub it over the damaged area.
When the acid filled Q tip contacts the calcite, there will be a little
bubbling take place as the calcite reacts with the acid to produce a little
carbon dioxide. Repeat this process as many times as needed to "clean" the
specimen. If the calcite has a dull surface and you want to make it shiny,
you can prepare enough of the diluted acid to imerce the entire specimen,
but you might want to dilute the acid more to 10 to 1 rather than 5 to one
as suggested above. When you put the specimen in the acid, the calcite will
react vigerously with the acid creating lots of carbondioxide bubbles. Leave
it in only a few seconds, take it out and wash it off with lots of water.
Let the specimen dry and observe the result. You may want to dip the
specimen again to increase the shine. You will quickly learn how long you
will need to leave the specimen in the acid to get the results you want. If
you are cleaning a lot of specimens or removing calcite to expose more
desirable crystals growing beneath the calcite the acid will be come
progressively weaker in reacting with the calcite. You will in this case
want to add a little fresh acid to maintain its reaction rate with the
calcite. You must be carefull not to use too much acid because if you do it
will become aparent to an experienced eye that the specimen has been acid
diped. They have a peculiar scaloped surface that you seldom see on natural
calcite speicmens. In addition, some dealers use various kinds of oil to
fill in cracks and enhance the look of specimens. The miners in Tsumeb used
to use a particular brand of sun block in spray cans to doctor up their
calcite specimens. They could quickly wash off the excess spray, but you
could always tell a treated specimen because the perfume in the spray would
linger and you could smell the treatment.
    The new Bolivian vivianite comes from a new "mine". You can quickly spot
it because it comes associated with a distinctive red sandstone that is
unlike any other locality that I know of. The crystals are not large and are
usually less than 2 inches but many of the crystals are a beautiful
transparent green and if it were not for its softness and great cleavage
many of the crystals would cut great gems. The full locality is the
Conutillos Mine, near Colavi, Potosi Department, Bolivia. The "mine" is
about a km down the canyon from the Canutillos mine and was discovered by a
mineral dealer who was waking down the canyon near some old mill workings
and noticed short black prismatic crystals in the wall of the canyon. These
turned out to be weathere vivianite and a little drilling and blasting
produced beautiful green fresh vivianites in small caveties in the red
sandstone. The miners from two of the local mines started mining the spot
for the specimens and started fighting over them and the "mine" was closed
for a while. We don't know how prolific the locality will be, but so far
several thousand specimens have been produced. The new mine has perhaps 100
meters of workings so far and how many more meters of tunnel will be made
chasing the specimens is any ones guess.
Rock