SCHOLZITE: & SEARCHING FOR THE EDIACARAN

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).

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.

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.

Scholzite crystals.  Width photomicrograph ~7 mm.

Scholzite crystals perched on matrix (goethite?).  Width of photo ~2.1 cm.

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.