Show Notes
2019 marks the sesquicentennial of Dmitri Mendeleev’s Periodic Table of the elements, which was not just an effective organizational scheme based on the atomic nature of the elements, but also a predictive tool regarding elements that had not yet been discovered in 1869.
The validity of the Table was proven in 1875. Mendeleev had predicted the existence of several elements, including one he called eka-aluminum. “Eka” is Sanskrit for “one,” thus meaning the unknown element would be one period down from aluminum. French chemist Paul Emile Lecoq de Boisbaudran identified a new element spectroscopically (as Mendeleev had foretold) in a sample of the mineral sphalerite (zinc sulfide). He named Element 31 gallium for his native France (Gallia in Latin) in 1875.
Mendeleev’s predictive table even allowed him to estimate properties of the unknown elements. When de Boisbaudran reported eka-aluminum/gallium’s density as 4.7 grams per cubic centimeter, Mendeleev suggested to him that he should recalculate it. When the new measurement was made, gallium’s density was found to be 5.9 g/cc, almost exactly as Mendeleev predicted several years earlier on theoretical grounds alone. Today the density of gallium is taken as 5.904 g/cc.
As described in my book What Things Are Made Of, gallium is used in tiny but critical amounts in solid-state semi-conductors in lots of high-tech devices, from computers and radios to cell phones and televisions. Gallium also finds its way into wireless technology infrastructure, integrated circuits, LED manufacture, and solar cells.
As it was when it was discovered, gallium today is found mostly in zinc and aluminum minerals and is produced by refining those ores, which are dominantly sphalerite for zinc and bauxite for aluminum. There’s typically only 50 parts per million or less gallium in such ores, but refineries can often separate it economically. In terms of crustal abundance, gallium is about the same as lead – but while lead easily forms common minerals that can be exploited economically, gallium’s properties mean that the few minerals it forms are extraordinarily rare. Instead, gallium gets incorporated into the structures of those zinc and aluminum compounds.
The United States is 100% dependent on imports for gallium, mainly from China (32%), United Kingdom (28%), Germany (15%) and Ukraine (14%). China is overwhelmingly the 600-pound-gorilla in terms of gallium, with more than 95% of total world production.
The sphalerite in this photo probably has some gallium in it, a few molecules at least, but I haven’t had it analyzed. The mineralization is hosted in limestones and cherty rocks of Mississippian age, deposited about 345 million years ago. The mineralization came in at some later time.
—Richard I. Gibson
More from History of the Earth
The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.
Mineral Monday + Tectonic Tuesday. Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
Epitaxy, from Greek words meaning “upon” or “above” and “ordered arrangement,” in minerals means crystals of one (or the same) mineral growing in a particular crystallographic position on another (or the same) mineral. It happens because the molecular spacings and orientations happen to be similar, allowing, even encouraging the crystal structure of the second, later mineral to mesh with that of the first. Some mineralogists might say that epitaxy requires the two minerals to be different minerals, but I do not – just two distinct generations of crystallization.
In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.