The Elegant Lavender Interior of the Gastropod Drupa forum

The gastropod Drupa Röding, 1798 is a Pleistocene to Recent genus, with about eight species and several possible subspecies. The type species [the original species used to define this genus] is Drupa morum Röding, 1798, commonly known as the “Purple Drupa, the Purple Pacific Drupa, or the Mulberry drupe snail.” The word “drupa” is Latin for ripen, in the sense of ripening of fruit. The Drupa shell does resemble a small berry (e.g., Mulberry).

 

Drupa morum Röding, 1798 from the Indo-Pacific area. The first image above is of the dorsal (back) side of the shell. The second image is of the ventral (front) side of the shell. The characteristic lavender color of this species is on present on one side of the aperture. The number "2" at the top refers to a later slide, which shows this same specimen in a series of other specimens.

Genus Drupa belongs to family Muricidae, and some specialists place the genus in the subfamily Thaidinae. Drupa is a predatory gastropod that prefers to eat worms, as well as some gastropods. The modern-day geographic distribution of Drupa is essentially restricted to between 35°N and 35°S (the area in "red," shown in the map below), in the Indo-Pacific (tropical) shallow-marine waters among coral reefs or rocky areas. Many of the species of Drupa are widespread within this region.

The species of Drupa are determined by several main morphologic features: the number of spiral rows of knobs on the last whorl (largest whorl), the shape of the knobs, the number and position of the labral teeth on the outer lip, the number and position of the columellar teeth along one side of the aperture (the main opening of the shell), and the color of the aperture. The labral teeth and the columellar teeth (both the oblique and straight ones) are labelled on the immediately following image.

The next two "strip images" show a growth series of six specimens of D. morum. The first one shows the dorsal side of each shell (numbered in increasing size), and the following strip shows the corresponding ventral side of each shell.

 Height range is 27.3 to 35 mm.

If you look carefully, you will see that the number of labral teeth and the number of columellar teeth, as well as their spacing, are not all the same on every specimen, even though the sizes of the specimens are similar. Nevertheless, all six specimens are Drupa morum. If the identification were based on only two specimens (numbers 1 and 6---see image below), however, it would seem that there are two species. That is why one needs to study as many specimens as possible. The outer lip of D. morum is especially prone to reabsorption of the calcium carbonate making up the shell, thus the details of the outer lip are not constant.

In the following image, specimen no.1 had reabsorbed some of the shell material making up its labral teeth along the outer lip of its shell. Specimen no. 6, which has all of its labral teeth, is shown for comparison.

Turquoise, an exquisite blue/green gemstone

 The word “turquoise” is an old French adjective, used to describe objects taken from the region of Turkey. One of these objects includes beautiful sky blue to green stones, which were originally mined in the Sinai Peninsula or over 5,000 years, thereby making turquoise one of the first gemstones to be mined. 

Today, turquoise is most commonly found in the southwestern United States, Mexico, China, Chile, Egypt, and Iran.  

Turquoise is a hydrated copper aluminum phosphate mineral. Its chemical composition is CuAl6(PO4)4(OH08)-4H20. Its composition can be variable, and even turquoise deposits in the same general area can have slightly different chemical compositions.

It is a secondary mineral that forms in arid or semi-arid regions, when rainfall leaches copper, aluminum, and phosphorus out of host rocks (plutonic and volcanic igneous rocks, or sedimentary phosphate beds). The leached material can also include impurities (strontium, lead, etc.). When the requisite elements are in the right combination, they get re-precipitated as nodules, encrustations, in veins, or in massive form. 

Two views of a nugget (5 cm width) of turquoise with “spiderweb” cracks:

Lastly, a view of a thin crust (2.5 cm length) of turquoise:  

At certain locales in the southwestern United States, turquoise can be found as the result of weathering of hydrothermal porphyry-copper deposits (PCDs). For more information about PCDs, see my previous Post.

Also, in the southwestern United States, turquoise typically forms within 100 feet of the ground surface. The shallow depth allowed Indigeous Americans to mine mineral in areas now known as Arizona and New Mexico. 

 Porphyry Copper Deposits

The word “porphyry” is a textural term [not a compositional one] referring to large-grained crystals (called phenocrysts) dispersed in a fine-grained (aphanitic) crystal groundmass. There is commonly a large difference in size between the tiny (can be microscopic) matrix material and the much larger crystals (>2mm). 

The above image is an example of a rock with porphyry texture: a rhyolite (volcanic igneous rock), 12 cm in high. This rock came from a river deposit of Mio-Pliocene age in southern California. This rock does not contain copper.

The next image is an example of pure native copper (50 mm maximum dimension). It was extracted from a porphyry copper rock at a copper mine near Morenci, Greenlee County, Arizona. 

Porphyry copper deposits (PCDs) account for the world’s largest source (about 60%) of copper. These deposits consist of disseminated copper (low to moderate grade [0.3 to 2 percent] copper and copper minerals in vein systems and in fractured rocks (breccias) with porphyry texture and containing copper formed from hydrothermal (heated) fluids that emanated from plug-like magma chambers in the upper crust (less than 5 to 10 km depth). These veins invade host rock, which can range from granitoid  porphyry intrusions (granites to diorites) and from adjacent volcanic wall rocks (rhyolites to basalts). Along with copper, other ores can be associated, like gold, silver, iron, molybdenum, lead, zinc, and tungsten.

The above two views are of the same hand-specimen sample of a copper porphyry from the Keweenaw Peninsula, Michigan, near Lake Superior. The native copper is disseminated in a conglomeratic sandstone/shale interbedded with rift-zone basalt (a volcanic rock). The maximum dimension of this hand specimen is 60 mm.

Porphyry copper deposits have formed throughout most of Earth’s history, but 90 percent of the known ones are Cenozoic or Mesozoic in age. They form mainly in dynamic tectonic areas: under big volcanoes and over suduction zones. They can form, furthermore, in continental magmatic areas along convergent tectonic plate margins or in island-arc environments. PCDs are found predominantly along the west coast of North America (from Alaska to Mexico), the Andes in South America, the Alps region in southern Europe, central Asia, the southwestern Pacific, and the east coast of Australia. 

 

An example of a continental magmatic area in southwestern North America is the one extending from Nevada to Mexico. An example of an island-arc environment is in Papua (province)  (shown below) is in the southwestern Pacific Ocean area. 

Porphyry copper deposits are currently the largest source of copper ores. Because the content of copper is very low, in many cases the ores are mined via huge open-pit from which massive amounts of rock are excavated and processed. One of the world’s largest and richest PCDs-mines is at Grasberg, Papau, southwest Pacific Ocean. The images below, which are from Google Earth (2020) show the location of this mine, at increasing magnification. In the second image, you can see a road spirally descending into the very large open-pit mine.

The main source of the information I used about PCDs is the very thorough paper by John, D.A., et al. 2010. Porphyry copper deposit model. United States Geological Survey Scientific Investigations Report 2010-5070-B. Chapter B. 169 pp. The pdf is available online for free.