作夢夢到的一份白蘭式 - 英檢

鐘乳石的補充...from Wiki (english)

Stalactites are formed by the deposition of calcium carbonate and other
minerals, which is precipitated from mineralized water solutions. Limestone
is calcium carbonate rock which is dissolved by water that contains carbon
dioxide forming a calcium hydrogencarbonate solution. The chemical formula
for this reaction is:[1]

CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)

This solution travels through the rock until it reaches an edge and if this
is on the roof of a cave it will drip down. When the solution comes into
contact with air the chemical reaction that created it is reversed and
particles of calcium carbonate are deposited. The reversed reaction is:[1]

Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)

An average growth rate is 0.13 mm (0.005 inches) a year. The quickest growing
stalactites are those formed by fast flowing water rich in calcium carbonate
and carbon dioxide, these can grow at 3 mm (0.12 inches) per year.[2]

Every stalactite begins with a single mineral-laden drop of water. When the
drop falls, it leaves behind the thinnest ring of calcite. Each subsequent
drop that forms and falls deposits another calcite ring. Eventually, these
rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda
straw" stalactite. Soda straws can grow quite long, but are very fragile. If
they become plugged by debris, water begins flowing over the outside,
depositing more calcite and creating the more familiar cone-shaped
stalactite. The same water drops that fall from the tip of a stalactite
deposit more calcite on the floor below, eventually resulting in a rounded or
cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as
hollow "soda straws." Given enough time, these formations can meet and fuse
to create columns.

Stalactites can also form in lava tubes, although the mechanism of formation
is much different.

[edit] Concrete
Experiment modeling drip stone formation in the lab.

Stalactites can also form on concrete, and on plumbing where there is a slow
leak and limestone (or other minerals) is in the water supply, although they
form much more rapidly there than in the natural cave environment
(description and experiments see literature).

The way stalactites form on concrete is due to different chemistry than those
that form naturally in limestone caves and is the result of the presence of
calcium oxide in concrete. This calcium oxide reacts with any rainwater that
penetrates the concrete and forms a solution of calcium hydroxide. The
chemical formula for this is:[1]

CaO(s) + H2O(l) → Ca(OH)2(aq)

Over time this calcium hydroxide solution reaches the edge of the concrete
and, if the concrete is suspended in the air, for example, in a ceiling or a
beam, then this will drip down from the edge. When this happens the solution
comes into contact with air and another chemical reaction takes place. The
solution reacts with carbon dioxide in the air and precipitates calcium
carbonate.[1]

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

When this solution drops down it leaves behind particles of calcium carbonate
and over time these form into a stalactite. They are normally a few
centimeters long and with a diameter of approximately half a centimeter.[1]

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