Life
is full of beauty. Notice it. Notice the bumble bee, the small child, and the
smiling faces. Smell the rain, [see the small minerals]and feel the wind. Live
your life to the fullest potential, and fight for your dreams. Ashley
Smith
The arsenates are perhaps my favorite group of
minerals, and along with the phosphates and vanadates, have received several postings in my Blog (i.e. copper arsenate: olivenite and clinoclase
and cornwallite; and copper, zinc arsenate: austinite; cobalt arsenate: erythrite;
lead arsenate: mimetite; nickel arsenate: annabergite). In these three groups arsenic (As) or
phosphorous (P) or vanadium (V) combine with oxygen (O) to from the arsenate
(AsO4), phosphate (PO4) and vanadate (VO4) radicals. Each of these radicals, with a negative
charge of 3- then combines
with a positive charged cation metal(s) and often with water (H2O)
or hydroxide (OH) to form a wide variety of minerals. Since the three radicals are approximately
the same size they often substitute for one another in a solid solution
series. For example, pyromorphite [lead
phosphate [Pb5(PO4)3Cl] is in solid solution
with mimetite [lead arsenate Pb5(AsO4)3Cl]. The latter mineral is usually a pale yellow
to yellow-brown color while pyromorphite is usually green to yellow-green in
color; however, intermediate stages in the solid solution series are known
(from work with XRD or EDS or other gizmos).
Each of these radical groups may also combine with a variety of metals
that often form solid solution series with each other. For example, erythrite [cobalt arsenate] is
in a complete solid solution series with annabergite [nickel arsenate] as the cobalt
cation substitutes for the nickel cation: Co3(AsO4)2-8(H20)
to Ni3(AsO4)2-8(H20). Therefore, it
is easy to understand the wide range, number and variety of arsenate, phosphate
and vanadate minerals when so many combinations of cations and radicals are
possible.
Many arsenate—phosphate—vanadate minerals are bright
in color, have easily observable crystals and are widely available at mineral
shows. Therefore, I am a sucker,
actually a buyer, whenever these minerals are located at shows (if the price is
right)!
The 13 July 2015 Blog Posting described pink erythrite
(cobalt arsenate) and green annabergite (nickel arsenate) and noted that a zinc
arsenate mineral called köttigite is the zinc analogue [Zn3(AsO4)2-H2O]
of both minerals. Therefore, I have been on the lookout for this fairly rare
mineral and was delighted when I was able to pick up three small specimens at
$1 each (a great price) collected from the Mina Ojuela, Mapimí, Durango, México.
Moore and Megaw (2003) described the Ojuela Mine as Mexico’s greatest
mineral locality with 117 species known from the deposit including the world's
finest adamite (in a gorgeous array of colors and habits), legrandite,
kottigite/parasymplesite, and paradamite, as well as superb specimens of
scorodite, hemimorphite, plattnerite, aurichalcite, rosasite, fluorite,
calcite, wulfenite and other species. Mina Ojuela is also the type locality for
paradamite, lotharmeyerite, metakottigite, mapimite and ojuelaite, and the
co-type locality for scrutinvite. The area
was first mined by Spanish invaders in 1598 and was commercially mined for 350
years. Today a few specimen miners still
haunt the drifts and adits.
Bob Jones, one of my heroes in the world of minerals,
noted in his 2011 book entitled The Frugal Collector that “of all the
species from the Ojuela mine, perhaps the least eye-appealing is köttigite,
which is a relatively dull blue-gray mineral.”
Hero or not, I do not agree with Bob’s opinion! I consider my three
specimens as quite appealing and beautiful with very nice sprays of prismatic
crystals.
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Spray of köttigite crystals showing a distinct color
variation from bottom (blue-gray) to top (clear). This variation may be due to the amount of
iron replacing zinc. The S is
water-clear selenite gypsum. The ? are
probably larger crystals of köttigite while
the “yellow” mass is unknown. Photomicrograph width FOV ~1.5 cm.
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The dark blue-gray center spray of köttigite is very
iron rich while a smaller spray to the left has rather clear crystals. The S is selenite gypsum while the G is
goethite. Photomicrograph width FOV is
~1.5 cm.
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Mixed color spray of köttigite and water-clear selenite gypsum (S) and
goethite (G). Width FOV ~1 cm.
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Köttigite, according to MinDat, should have colorless
crystals but due substituting chromophores it shows a variety of colors ranging
from red to red-orange to brown to rose pink to gray to gray blue (and probably
more). The prismatic crystals have a
silky to waxy luster, are soft (2.5-3.0), are flexible and are transparent to
translucent. The mineral may also appear
as encrusting rather than forming nice prismatic crystals.
Köttigite is formed in the oxidized zone of
polymineral ore deposits by the alteration of primary hypogene minerals
skutterudite [CoAs2] and sphalerite [ZnS]. Köttigite is the zinc
analogue of, and in solid solution with, erythrite (cobalt arsenate) and
parasymplesite (iron arsenate). In fact,
as köttigite obtains more and more iron the crystals become gray to light green
to blue to greenish black and as I understand the situation, it is almost
impossible to visually distinguish between köttigite and parasymplesite in
many/most specimens. Therefore, some
technical journals use a hyphenated mineral name unless a chemical analysis has
been completed. In addition, köttigite (Monoclinic Crystal System) is a dimorph
(same chemical formula but different crystal system) with metaköttigite
(Triclinic Crystal System).
I have learned much from this little project,
especially about how the swapping of metallic cations and substitutions of the
radicals can produce an amazing number of minerals. But most importantly, I learned how to
produce, in WORD, a diacritical mark called an umlaut. You have all seen an umlaut, those two little
dots over a vowel as in köttigite indicating “a partial assimilation to a
succeeding sound. Used primarily in German.” If you really want to pronounce köttigite
correctly go to www.webmineral.com and
type in the name and look for the voice button.
I tried to learn German one time about a decade
ago—spent six weeks taking a class in Germany.
This exercise was a tough assignment and about the only actions learned
were phrases to order dark beer, sausage and pig knuckles, and bread. On the final day of class our instructor took
us to a nice outdoor café for lunch and we needed to converse with the staff—in
German. OK, I ordered food and beer but
what about more conversation? So with my
very limited vocabulary I asked the wait staff if she would like to go dancing
on the beach in the moonlight! That was
about the only sentence I could muster except one telling her about my dog and
his name. I think my instructor was
humiliated; however, I did receive an extra desert from the staff!
REFERENCES
CITED
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Moore, T.P. and P.K.M. Megaw, 2003, Famous mineral localities:
The Ojuela Mine, Mapimí, Durango, Mexico: Mineralogical Record, v. 34, September.
As for the beauty of köttigite I like the words of
Tadao Ando: you can't really say what
is beautiful about a place [mineral], but the image of the place [mineral]
will remain vividly with you.
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