In my continuing search for nifty arsenates, vanadates,
and phosphates I ran across a specimen of the rare calcium zinc phosphate
mineral, scholzite [CaZn2(PO4)2-2H2O)]. I was attracted to the specimen for a couple
of reasons: 1) I had recently finished reading an article describing the many
uncommon/rare phosphates (Type Locality for scholzite) collected from
Hagendorf, Bavaria, Germany; 2) the phosphate minerals from the area are
similar to the minerals collected from the lithium-bearing phosphatic
pegmatites in the Black hills of South Dakota; 3) the numerous glassy, gemmy,
terminated crystals of scholzite were impressive. The Hagendorf mines generally
produced feldspar from the late 1800s until the 1980s (seems similar to Black Hills
feldspar mines). The pegmatites are
mostly zoned intrusive bodies (~300 Ma) that were intensively weathered in the
late Tertiary (~4-5 Ma) (Dill, 2009). As
I understand, scholzite can form as a primary mineral in late stage phosphate
mineralization; however, it is more common as a secondary mineral in the
oxidation zone of zinc- and phosphate-bearing pegmatites.
The specimen I purchased, however, came not from
Germany but from another famous location in one of the Flinders Ranges in southern
Australia north of Adelaide. I am not
very familiar with Australia but did recognize the locality, not due to the
mining activity, but because of a subsection of the Flinders Ranges called the Ediacaran
Hills. Many decades ago, when I enrolled
in Historical Geology, the base of the Cambrian Period (then established at ~600
Ma) was defined as the “beginning” of multicellular life. This confused many learned paleontologists
since Cambrian trilobites, among other animals, first appeared as diverse and
complex animals. In fact, they crawled
around and had eyes---something one might not associate with a newly evolved
group of animals. In order to explain
this sudden appearance of life, American paleontologist Charles Wolcott coined the
term Lipalian Interval as a period of time between the younger Precambrian
rocks and the oldest Paleozoic (Cambrian) rocks. Wolcott believe the youngest Precambrian
rocks (the time when animals evolved) had been eroded and no longer existed, or
perhaps had not been discovered. This
position was easy to believe since in so many areas in the world, especially in
the United States, the earliest Paleozoic rocks, commonly transgressive marine sandstones,
unconformably rest on top of igneous or metamorphic Precambrian rocks. The text
books of the time were full of photos of the Lipalian Interval, especially of
rocks in the Grand Canyon where the regional unconformity separates rocks of
the Cambrian Tonto Group from the Precambrian tilted and folded Grand Canyon
Supergroup (and below that the basement rocks of the “Vishnu Schist”-- probably
several different rock units in the schist).
John Wesley Powell, in his travels through the Grand Canyon, referred to
the Precambrian-Cambrian unconformity as The Great Unconformity (essentially synonymous
with The Lipalian Interval).
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| The Great Unconformity as described by John Wesley Powell during his 1870s journey down the Colorado River through the Grand Canyon. Geologists then believed that a major unconformity separated Precambrian rocks from the overlying Paleozoic rocks on a world-wide basis. Sketch courtesy of the US National Park Service. |
The other unknown, or misunderstood, geological theory
during my undergraduate years was the concept of “Continental Drift” (today
known as Plate Tectonics). We dutyfically
studied Miogeosynclines and Eugeosynclines and really never understood how
these features formed or operated. In
those days, most undergraduate students never questioned the wisdom of their
instructors or the textbooks authors. We
were just beginning to hear about “drifting continents,” a theory developed in
the first half of the century but certainly did not understand what mechanism
drove the continents to “drift around.”
Then in the early 1960s seafloor spreading was validated and suddenly, a
mechanism was available to move continents.
However, I really did not learn much about plate tectonics until enrolling
in graduate studies, and even then, some of the instructors were non-believers.
Then “things” began to fall in place. Geologists started to better understand plate
tectonics and assigned the name active
plate margins to areas where the plates were moving “forward” and then colliding
with other plates resulting in “mountain building! Passive plate margins were the trailing edges of plates where tectonic
activities were less active and where large volumes of sediments from entering
streams, and deposition of marine rocks, were piling up on the wide continental
shelves. A great modern example in North
America is to look at our west coast where mountain building, faulting, volcanoes and earthquakes indicate an active plate—the continental plate is
banging into and overriding an oceanic plate(s). The east coast provides an example of a
passive plate where wide oceanic shelves are collecting sediments and lime
rocks. Of course, conditions change over
geologic time and the badly eroded Appalachian Mountains were produced along an
active plate margin in the late Paleozoic Era.
 |
| Active (west coast US) and passive (east coast US) plate margins. Diagram courtesy of geologycafe.com at MiraCosta College. |
And guess what?
In some of these passive margins around the world deposition continued
from the latest Precambrian up into the Cambrian (now established as beginning ~542
Ma)--for example the Wood Canyon Formation in the Death Valley Region. And the second guess what--- multicellular
animal fossils were located in these latest Precambrian rocks. The animals did not have skeletons but
certainly had complex body plans---“like” jellyfish, worms, arthropods, fronds,
bags and lots of unknowns! Did these Precambrian
organisms and their communities flourish into the Cambrian? Probably not as they were replaced by
skeletal animals in the great Cambrian Explosion, and perhaps even provided
food for the early Cambrian animals. But
again, the question remains---what about the skeletal animals of the
Cambrian? They seem not closely related
to the non-skeletal animals of the latest Precambrian, so……..? One of life’s persistent questions.
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| Dickinsonia sp. from the Ediacaran Biota. Public Domain photo. |
Although fossils of these latest Precambrian multicellular
organisms have now been found on every continent, geologists have named the
community the Ediacaran Biota after localities in the Ediacara Hills of south
Australia in the Flinders Ranges. These
fossiliferous sedimentary rocks were deposited along the passive margin of a “continent”
that composed part of the Precambrian Supercontinent Rodinia. In addition, in 2004 the International Union
of Geological Sciences named the last period of the Neoproterozoic Era of the
Precambrian (latest Precambrian) the Ediacaran.
Although absolute dates remain somewhat uncertain most stratigraphers
place the beginning of the Ediacaran Period at ~635 Ma, commencing after the
end of the global Marinoan Glaciation. The Ediacaran was the first officially
approved geological period in 120 years.
The specimen of scholzite in my collection came from
the Reaphook Mine in what Hill and Mills (1974) termed “near-surface
mineralized zones in unmetamorphosed sediments of the Lower Cambrian Parachilna
Formation…in the Flinders Ranges, South Australia. The mineralized zones have
resistant ferruginous and manganiferous cappings, which grade downwards into
complexly fractured phosphatic pebble conglomerates, sandstones, and
siltstones. They seem to have developed as a result of the action of
groundwater causing near-surface enrichment of manganese, iron, zinc, and phosphorus
in fractured and faulted zones in the Parachilna Formation.” Scholzite at Reaphook is associated with a
number of other phosphate-rich minerals and the Mine is the Type Locality for another
calcium zinc phosphate, hillite [Ca2Zn(PO4)2-2H2O].
The Flinders Ranges of South Australia have a long
history of mining for zinc, silver, barite, lead gold, uranium and others. However, most of these mineral deposits are
located in the Northern Flinders Ranges and the Reaphook Hill is in the
southern part of the Ranges. About the
only reference that I could locate about Reaphook is MinDat.org: “a zinc and
phosphorus-rich deposit…it [was] mined for a few years for mineral specimens
(mostly Scholzite).” In addition, the 22
collected minerals listed by MinDat do not include anything that I would call a
valuable ore mineral.
Scholzite (Orthorhombic) is an interesting mineral and
could be mistaken for other vitreous, transparent to translucent, white to
colorless, soft (3.0-3,5; Mohs) phosphates such as its dimorph, parascholzite
(Monoclinic). Scholzite commonly appears
as radiating blades of crystals with pointed terminations, or as less-acicular
tabular crystals. The mineral’s rarity
in being restricted to zinc- and phosphorous-rich rocks is an important guide
to initial identification based on physical appearance. Any in-depth identification probably requires
the use of sophisticated electronic gizmos. I also find it interesting that
this rare mineral is also found in the pegmatites of the Tip Top Mine in Custer
County, South Dakota.
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| Scholzite crystals. Width photomicrograph ~7 mm. |
 |
| Scholzite crystals perched on matrix (goethite?). Width of photo ~2.1 cm. |
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| Photomicrograph scholzite crystals. Width photo ~7 mm. |
For my collection of somewhat rare and interesting
minerals, I was happy to snag a small specimen of scholzite. And I became even more excited to dredge up fond
memories of my early learning about the existence of Ediacaran (AKA Eocambrian)
rocks and fossils, not to mention passive plate margins and seafloor spreading.
REFERENCES
CITED
Dill, H.G., 2009, The Hagendorg-Pleystein phosphate
pegmatites (NE Bavaria, Germany) – A mineralogical, chronological and
sedimentological overview: Estudos Geologicos, vol. 19, no. 2.
Hill, R.J. and A.R. Milnes, 1974, Phosphate minerals
from Reaphook Hill, Flinders Ranges, South Australia: Mineralogical Magazine,
vol. 39.
