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| CALCITE PSEUDOMORPH AFTER GLAUBERITE. LENGTH ~5 CM. |
One of the more interesting groups of minerals are the pseudomorphs,
or false form minerals--essentially a new mineral recrystallizes and replaces the
original mineral. During this chemical
change the replacing mineral takes on the crystal form of the original mineral
and that crystal form commonly is atypical for that particular replacing mineral! That is, the shape of the original mineral is
maintained by the replacing mineral.
For example, both azurite and malachite are quite
recognizable copper minerals with azurite being blue in color while malachite is
a bright green. Both minerals are copper
carbonates with azurite (Cu3(CO3)2(OH)2))
crystallizing usually as prismatic crystals while malachite (Cu2CO3(OH)2) crystals are
often slender
prisms. Azurite is unstable and with
weathering some of the carbon dioxide (CO2) chemically changes into
water and the +++copper cation becomes ++copper cation. This seems a fairly complex chemical change
for such beautiful minerals!
Near Lake George
in the pegmatites of the Pikes Peak Batholith, hematite is a pseudomorph after
siderite. Hematite is an iron oxide (Fe2O3)
and often a weathering product, in this case of siderite, an iron carbonate
(FeCO3) with a rhombohedral form.
At many rock and mineral shows
vendors will display glauberite pseudomorphs that were collected from near Camp
Verde, Arizona, north of Phoenix. At
this locality exposures of the Verde Formation are well-exposed in an abandoned
salt mine. The Verde was deposited in a
large lake that occupied a tectonic basin in central Arizona during the
Pliocene and Pleistocene epochs, approximately two to eight million years ago (Ayres,
2009). Near the end of the lake cycle
the water became quite saline and evaporitic minerals such as halite (sodium
chloride-- NaCl) and glauberite (sodium calcium sulfate-- Na2Ca(SO4)2
were deposited.
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| THE "SALT MINE" NEAR VERDE, ARIZONA. |
I am uncertain about the
chemistry but sometime in the last two million years carbonates, calcite (CaCO3)
and/or aragonite (CaCO3), or a sulfate, gypsum (CaSO4-2(H2O)), replaced
the unstable glauberite as a pseudomorph.
I presume that groundwater percolating through the sediments and rocks
provided the appropriate replacing elements.
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| GYPSUM PSEUDOMORPH AFTER GLAUBERITE. LENGTH ~10 CM. |
I also recognize
there is a name similarity between glauberite and Glauber’s salt, the latter being
a sodium sulfate decahydrate (Na2SO4-10H2O), for which the mineral was named. In past years some glauberite was mined to
produce Glauber’s salt, a substance used in the chemical industry. Glauberite itself is mostly colorless to
cream to gray in color, rather soft at 2.5 on the Moh’s Scale, and has a white
streak. Glauberite is impressive due to the
unmistakable, large, well-formed, tabular- to wedge-shaped crystals that define
the mineral. This characteristic shape
seems enough to distinguish the mineral from others. However, as noted above, there is a
problem--glauberite is unstable and often is replaced by other minerals producing
pseudomorphs and these “false form minerals” seem more common in the record
than true glauberite crystals!
In my collecting at the salt mine
I was able to secure: 1) a beautiful large specimen of several crystals of
gypsum that were pseudomorphs of glauberite, and a “typical” crystal of calcite
pseudomorph after glauberite. I also
collected a transparent and non-crystal specimen that an Arizona geologist
identified as thenardite (a mineral that was unfamiliar to me). Thenardite is a sodium sulfate, Na2SO4,
that also precipitates in evaporitic lakes and playas. Interesting, but perhaps confusing to
non-geochemists, are the facts that thenardite: 1) is the salt of sulfuric
acid; and 2) and becomes Glauber’s salt with the addition of water!
A couple of other interesting comments about glauberite
might be in order. In New South Wales,
Australia, opal (SiO2-nH2O)
is found as a pseudomorph after glauberite. At Watchung, New Jersey, both prehnite (a
calcium, aluminum phyllosilicate, Ca2Al(AlSi3O10)(OH)2))
and quartz (SiO2) occur as pseudomorphs after glauberite in basalt
cavities.
I am somewhat out of my realm of comfort here since
I am not a mineralogist or geochemist and do not fully understand some of the
processes taking place during the formation of pseudomorphs. However, the glauberite pseudomorphs are
quite interesting and make excellent display specimens. For additional information on the Camp Verde
specimens please see Ayres (2009) or Thompson (1983).
REFERENCES
CITED
Ayres, S., 2009, The Verde Formation: A Story That
Holds Water: Verde Independent (newspaper), November 18, 2009.
Thompson J. R., 1983, Camp Verde Evaporates: Mineralogical Record, Vol. 14 No. 2, p.
85-90.