CHANNELING A MOTHER ROCK
Rock&Gem Magazine|July 2021
Mineral Constituents of the Chert Complex
REBECCA SOLON

Picasso Chert from Burro Creek, AZ, collected by Arizona State University Geology Professor and Author Stan Celestian.

In the March 2020 issue of Rock & Gem, in my article entitled Cherishing Chert: A Mother Rock for the Ages, I discussed the ancient origins of chert as a building block of Earth’s early ocean basins. I illustrated my narrative with two famous formations, the Kaibab Cliffs of the Colorado Plateau and the banded iron formations (BIFs) of Mingus Mountain in the Black Hills region of north-central Arizona.

These ancient deposits crop out at the top of Mingus Mountain and are the source of the hematite and jasp-chert BIFs of the Verde Valley. Chert is a name that is often used interchangeably with another impure opaque variant of chalcedony known as jasper thus the use of the term “JaspChert.” The unique rocks described in this article combine distinguishing characteristics of both varieties and constitute minerals of the chert complex.

CHERT-BASED MINERALS COMBINE ALL ROCK CLASSIFICATION CATEGORIES

The chert-based Kaibab and BIF rocks containing calcium and magnesium-carbonate and iron oxides constitute just a few chert minerals commonly found in Arizona and are now known worldwide. These mineralized rocks establish the complex of chert formations that encompass Earth’s fundamental stratigraphic units as defined by the Geologic Time Scale, and the study of physical geology.

By using the term “complex” this author intends to define units of chert formations further as composed of rocks of two and often three of the standard classifications of sedimentary, metamorphic, and igneous rocks. The chert complex contains a myriad of mineral inclusions that are found globally within these stratigraphic features, both as trace elements and as complex combinations of minerals.

Chert is normally considered to be a sedimentary rock. But there are now multiple variations being scientifically studied and shown worldwide that contain multiple minerals and patterns. Most chert rocks contain some quartz which accounts for its dense microcrystalline or cryptocrystalline composition. Concerning its typically high quartz content, chert can be an organic or inorganic precipitate or a replacement product in sedimentary rocks.

According to the reference page https://mrdata.usgs.gov/ geology/state/sgmc-lith.php?text=chert Chert is defined as “A hard, extremely dense or compact, dull to semivitreous, microcrystalline or cryptocrystalline sedimentary rock, consisting dominantly of interlocking crystals of quartz less than 30 micrometers in diameter.”

Chert is normally classified as a biogenic or organic form, but there are numerous examples of inorganic chert rocks from Arizona.

My mentor Cliff Montgomery sold me several beautiful, tumble-polished pieces of Arizona pastelite chert, which I made into interchangeable necklaces. A related species collected by Stan Celestian is a highly patterned pastelite variety from Burro Creek, Arizona, that he has named “Picasso Chert.” His photo collection features this specimen and includes one with differential weathering in a rough and unpolished chert specimen from this locality. The unique weathered surface is described as having a rillensteine pattern. The Dictionary of Geological Terms defines “rillensteine” as “Tiny solution grooves, of about one millimeter or less in width, formed on the surface of a soluble rock [from German, meaning ‘rilled rock’]”.

In 2009 an Arizona Geological Survey Team discovered an opaline variety of chert layered within a travertine formation in the McDowell Sonoran Preserve as a desert precipitate in Scottsdale, Arizona. According to the authors, “Limestone has not been identified previously in the McDowell Mountains or in the metro Phoenix area … The limestone deposit overlies or rests on a larger area of metamorphic rhyolite rock. The meta-rhyolite, a rock rich in silica, likely is the source of the mineral constituting the chert.”

Investigators prepared a thin section of a sample for photo-micrographic examination. The study team identified these geologic events; 1) original travertine limestone deposition; 2) formation of orbicular (concentrically layered spheroidal) chert and possibly dissolution of limestone; followed by 3) coatings of calcite, silica, and possibly algae in cavities; and finally, 4) void-filling by coarse calcite in a water-saturated environment.

The McDowell Sonoran investigation of a deposit of opalized chert illustrates a prime precipitation event by silica and the mineral SiO 2. Opal is hydrous silica (SiO 2 ·nH 2 O). Technically, opal is not a mineral because it lacks a crystalline structure, so it is typically referred to as a mineraloid. This opaline variety of chert was created in a natural opaline formation as a part of a unique limestone depositional event.

PROGENITOR OF DURABLE MOUNTAIN VISTAS

Chalcedony and quartz minerals play important roles as strengthening components that produce the nearly indestructible varieties of chert that have outlasted many geological eras. One good example is the Mogollon Rim, a geological feature that skirts the northern half of Arizona and forms the southern edge of the Colorado Plateau in Yavapai County, ending near the border with New Mexico. The Mogollon Rim is a complex assemblage of mineral constituents and is representative of the chert complex of Arizona.

These massive Paleozoic limestone-chert formations are remnants of coral reef outcrops of the Mogollon Rim seabed deposits.

Now at over 6,000 feet elevation in the Payson-Pine rimstone region, the Mogollon highlands were originally part of a vast inland sea 225-280 million years ago — the Triassic-Permian periods. Since that time, the original seafloor now sits at a much higher elevation through uplifting from plate tectonics, reaching 8,000 feet at its maximum height.

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