In the last Blog posting I described a couple of
arsenates that came from the famous Ojuela Mine near Mapimi in Mexico. But, in doing so, I came across a couple of interesting
carbonates in my collection from the mine: 1) rosasite, a copper zinc carbonate
hydroxide [(Cu,Zn)2(CO3)(OH)]; and 2) aurichalcite, a
zinc copper carbonate hydroxide [(Cu,Zn)5(CO3)2(OH)6]. Aurichalcite has a Zn:Cu ration of about 5:2
while rosasite has a Cu:Zn ration of about 3:2.
Both of these minerals are found in the oxidized zones of copper and
zinc ore deposits; the primary minerals (supplying the zinc and copper) were
probably sulfates, such as chalcopyrite and sphalerite, that ultimately broke
down and moved around, usually toward the surface, in hydrothermal solutions.
As the percolating hot solutions reached overlying beds of carbonates the
acidic solutions ran into volcanic rocks that blocked their upward movement and
forced a horizontal movement along faults and joints. The solutions also dissolved part of the
carbonates and created additional spaces for minerals to form—all sorts of copper
and zinc rich minerals were competing for space.
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| Rosasite (R) botryoids and an encrustation of extremely tiny crystals. Width FOV ~1.0 cm. |
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| Nice rosasite (R) botryoid,( ~3.0 mm) with sub-millimeter clear calcite rhombs (C). |
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| Rosasite "balls" scattered over a black matrix (Camera makes it appears to have a metallic luster) that may or may not be chalcophanite. The beautiful and gemmy blue fluorite makes a nice addition (width upper fluorite ~3 mm). |
Rosasite (copper rich) is a nice “blue” (sky blue to
blue green) color that has been described as a “soft blue.” It is more
translucent than transparent and has a hardness of around 4.0+ (Mohs). The luster can be vitreous but in encrusting,
fibrous or botryoidal forms it is more like velvet or silk. Rosasite does have a light blue streak. At
Mapimi many rosasite specimens are botryoidal in nature, associated with
calcite, and form on a “limonite” matrix.
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| Aurichalcite (C) acicular crystals (all about 4 mm in length) interspersed with calcite crystals (C) perched on a "limonite" gossen (G). |
Aurichalcite, the zinc rich “cousin” of rosasite, also
has the light blue to blue green soft color but is more transparent than
translucent (as opposed to rosasite).
The mineral usually appears as a delicate, velvety coating of soft
acicular crystals with a silky luster (as opposed to vitreous). In fact, these two characteristics appear, at
least to me, as the best distinguishing features—the soft (1.0-2.0 Mohs) needle-like
crystals, often tufted in sprays, are easily moved around and crushed by a
sharp probe.
Both rosasite and aurichalcite are common minerals and
found in many localities; however, they never seem to occur in large amounts.
Jones (2011) noted that “the zinc that is released by weathering is grabbed up
to form several secondary zinc minerals: adamite, smithsonite, hydrozincite,
hemimorphite, and descloisite, to name a few.”
Therefore, competition for the zinc is the name of the game.
I have always been fascinated by the creation of
secondary oxidized minerals whose precursors were completely different
minerals. Just amazing. Sort of like the fact that old geologists never die, they just recrystallize.
REFERENCES
CITED
Jones, Bob, 2011, The Frugal Collectors Volume 1:
Ventura, CA, JMiller Media/Miller Magazines Inc.