POLLUCITE, CESIUM AND THE BUNSEN BURNER

Many times, while doing some research on specific minerals and their localities, I stumble across some little serendipitous facts.  So, it was for this offering.  There are not many minerals in this world that contain significant cesium (Cs) as the cation (positive charge) ---pollucite [(Cs,Na)₂(Al₂Si₄O₁₂)-2H₂O] is the major and best known example.  I knew that little tidbit but not much else about this relatively unknown element except cesium is used in oil well drilling muds, atomic clocks and a one radioisotope is important in the treatment of some cancers.

But wait, there is a whole lot more that makes an ole scientist like me smile.  I suppose many readers of these postings remember something about their high school or college chemistry course.  Maybe not much, but a little.  In fact, if I said what is the first thing that pops into your head when someone shouts out “chemistry class”—what would it be?  I took a quick survey of 10 people and 5 said something like “bad grades, tough class, etc.”, 1 said “blowing up sodium” but 4 agreed with me and said, “Bunsen burner.”  Yep, those little gas burners that are sitting on chem tables in virtually lab in the country.  A small rubber hose connected the burner with a natural gas outlet and a striker was used to light the apparatus although paper matches also worked.  All sorts of goodies were heated up in the lab by using those ubiquitous burners (some of the heatees were not listed in the lab book!).  If you were like me, I never gave a thought as to their origin. For all I knew they bred and hatched in the storeroom only to be brought out in the open at the beginning of each semester—"Nelson you have burner #24, don’t lose it.” 

A "modern Bunsen burner, gas intake on the left, needle valve on the right, and air regulation around base.  Public Domain photo.

 

 

 

 

When I escaped chemistry (thank God since I was not going to cut it as a chemist) I moved into geology and those nifty little burners were checked out for mineralogy class. Just remember that back in the early 1960s students did not have access to lots of electronic gizmos and we used fairly straightforward tests to try and identify unknown minerals.  One of these simple tests (although the reasoning behind the test is quite complex) was called a flame test.   So, we had these metal doohickeys that sort of looked like a plastic tooth flosser except the wire was some type of a nickel-chrome alloy (platinum could be used but was more expensive).  We ground up the unknown mineral is a clean mortar and pestle, dipped the wire in hydrochloric acid, burned off any crud by holding it in the flame of a Bunsen burner, dipped it in acid again and then into the ground up mineral and then into the fire.  Certain elements imparted a color to the flame.  I don’t remember much except that sodium was an orange color, potassium was pink, and copper was blue-green. But, no questions were ever asked about the origin of Bunsen burners!

Well, now that you might be interested, the University of Heidelberg, in 1851, hired one chemist by the name of Robert Bunsen.  Bunsen did not like the current burners is use since they produced much soot and lots of light; burners needed low lumens (brightness) so that colors could be seen in the flame test. So, Bunsen had an idea and took his plans over to a mechanic (Peter Desaga) working for the University—a really good move. Together they “invented” the Bunsen/Desaga burner that had, among other things, adjustments for regulating air intake before combustion.  Somehow Desaga’s name was dropped over the years and the Bunsen burner was born---but in a mechanic’s shop and not the storeroom.

The early spectroscope of Gustav Kirchhoff.  Note flame to left, prism in the center and eyepiece on the left.

Earlier in his career, Bunsen had meet a physicist by the name of Gustav Kirchhoff when both were working at the University of Breslau.  After taking the professorship at Heidelberg in 1852, Bunsen soon secured a position for his friend Kirchhoff.   After seeing the wonders (adjustment of air intact could produce a hot blue flame) of the newly developed Bunsen burner Kirchhoff thought perhaps colors produced by the mineral flame tests could further be refined by looking at emission spectral lines (essentially the colors you see when sunlight is directed through a prism) through a prism.  Each element emits a light of a specific wavelength (and colors have wavelengths).  Short story is that the produced spectra (when examined through the prism) could be used to identify chemical elements and the science of spectrography was hatched (physicists might argue that Sir Isaac Newton published a paper in 1672 in which he described the spectral colors of sunlight when directed through a prism). A year or so later the two lab partners discovered and identified two new elements, cesium and rubidium, as they examined mineralized spa water.  For cesium, the blue-colored spectral emission lines (of cesium) did not correspond with lines of any known element (as currently known). So, they named one of their new elements cesium after the Latin word for blue, caesius.

That then is the short story of how a little gas burner led to the almost immediate discovery of two new elements and then opened an entirely new field of chemistry (spectrography) that in turn led to the discovery of numerous new elements by various chemists. So next time someone asks—what is the one thing you remember about your chemistry class?  We all shout Bunsen burners! 

During one summer in Germany I was able to visit Heidelberg, the work home of Robert Bunsen.  Photo Public Domain.

So, was there any use for the newly discovered cesium?  Not much in the early days but then chemists discovered that cesium easily and readily combines with oxygen.  Therefore, the initial major use was to cleanse oxygen and trace gases from vacuum tubes.

Continuing with our chemistry lesson, look at a Periodic Chart of the Elements and locate the element Hydrogen (in the far upper left corner).  Directly under the hydrogen are listed the Alkali Metals: lithium (Li), sodium (Na), potassium (K), rubidium (Ru), cesium (Cs) and francium (Fr) related by being highly reactive, soft, shiny, highly malleable (can pound it into thin sheets) and ductile (can form a thin wire), have only one electron in their outer shell, and do not occur freely in nature (always combined with something).  Note that some of these alkali metals are important rock formers—sodium and potassium for example, but something like francium is only found in trace amounts in some uranium minerals.

Cesium is sort of a “strange” element, in several ways. For one, it is the softest known element coming in at a whopping 0.2 on the Mohs scale of hardness.  In addition, it is one of only five elements (mercury, bromine, cesium, gallium and rubidium) that are liquid at around room temperature (Cesium melts at ~83^OF).  Cesium is not an element to play with since it ignites spontaneously in “normal” air and “blows up” (like sodium) in reacting with water. It is a waxy silver to gold metal that actually has a number of uses.

Cesium formate (formic acid and cesium salt: CHCsO₂) brine is used as a drilling mud in deep oil wells.  The mixture is extremely dense and helps “float” rock and mineral chips to the surface during the drilling process.  In more shallow wells something like bentonite (a clay) and water are used as the drilling fluid while the very expensive cesium formate is reserved for the deep wells.  In fact, the cesium formate drilling fluid returning up hole is filtered and sent to the mud pit.  It is so expensive that about 80-85% of the used fluid can be recycled.  It is my understanding, after talking to a well site geologist, that the expensive cesium formate brine may be rented from a mud company.

I don’t really understand this information so will just quote www.livescience.com: Cesium is incredibly accurate at timekeeping and is used in atomic clocks. The official definition of a second is the time it takes for the cesium atom to vibrate 9,192,631,770 times between energy levels. www.MinDat.org states a beam of energy is shined on a very pure sample of cesium-133. The atoms in the cesium are excited by the energy and give off radiation. That radiation vibrates back and forth, the way a violin string vibrates when plucked. Scientists measure the speed of that vibration. The second is officially defined as that speed of vibration multiplied by 9,192,635,770. s. Cesium-based atomic clocks lose one second per 100 million years.  I could never build an atomic clock and am perfectly happy with my Timex©

Cesium has only a single naturally occurring stable isotope and that is Cs-133. So, now everyone think back to the old days of chemistry class. Every atom (didn’t the text call them the building blocks of the universe) consists of heavier protons and neutrons in their center (the nucleus) surrounded by a cloud of electrons in the outer shell (the diagrams reminded us of planets orbiting the sun).  Each nucleus of atoms in a specific element has the same number of protons and that number is called the atomic number—cesium has 55 protons (the atomic number then) and the number of protons also equals the number of electrons; this assumes an ordinary neutral atom.  The atomic mass of cesium is ~133 which means that cesium has 78 neutrons (55 + 78=133) in its stable form and therefore I write such as Cs-133. Although each element has a stable number of protons (and electrons), the number of neutrons may vary, and these forms are called isotopes.  Cesium has isotopes ranging from Cs-126 (55 protons plus 71 neutrons) to Cs-139 (55 protons plus 84 neutrons) and perhaps more.  In cesium the only naturally occurring form is Cs-133 and all others are “man-made,” usually in nuclear power plants or fission-based explosives (perhaps in some planetary explosions but that is above my pay grade).  

  

One artificial and radioactive isotope of cesium, Cs-137, has very important uses in the medical field.  Small amounts (a seed and usually with an isotope of iodine) are implanted (brachytherapy) in some forms of “cancer” and begin to emit radiation.  The goal is that the radiation will “kill” the surrounding cancerous cells without harming other parts of the body. 

One of the more unique uses of Cs-137 is to track “ownership” of crude oil running through common pipelines.  Many petroleum producers use a shared pipeline to transport their individual crude to refineries.  So, as a new batch of crude is pumped to the conduit a small amount of radioactive Cs-133 is added and the isotope and the total amount of crude can be determined as the crude reaches its final destination (usually the refinery).

Another little factoid is that radioactive cesium is produced in nuclear power plants (and now captured we hope) but was produced in the atmosphere during the early days of “atomic/hydrogen” bomb testing.  The radioactive isotopes settled out of the atmosphere into the soil and today can be located in soil profiles where it may be used to study sediment transport.  Scary, but the better news is that Cs-137 half-life (half of the material decays to barite) is ~30 years so decay is fairly rapid. The bad news is that Cs-137 is a major radioactive contaminant at both Chernobyl and Fukushima Daiichi disaster sites.  The interesting news is that Cs-137 was not produced on the planet prior to 1942 (the Chicago Pile-1 under the football stadium at the University of Chicago).  Therefore, any containers sealed prior to 1942 would not contain traces of Cs-137.  I understand that checking for the presence of Cs-137 is a way to nab counterfeit “antique” wine! 

The world's first nuclear reactor, the Chicago Pile-1 constructed under the football stands at the University of Chicago.  Public Domain photo.

So where does cesium “come from?”  MinDat lists 18 valid minerals where cesium is the dominant metal cation.  However, pollucite [(Cs,Na)₂(AlSi₄O₁₂)-2H₂O] is the major ore mineral of cesium and may contain 40%+ of the element. Pollucite is a member of a very large group of minerals termed the zeolites that are aluminosilicates containing water.  In fact, zeolites can absorb large amount of water and therefore have hundreds of industrial uses; however, pollucite is much too rare to be used as an absorbent, catalyst or filter.  Common zeolites include the ubiquitous apopolite from India---seen at every rock and mineral show-- stilbite, heulandite, and others. Pollucite is also in solid solution with analcime as the sodium replaces the cesium.

Crystalline pollucite looks very similar to the zeolites listed above and often has a vitreous luster but feels waxy or greasy, is fairly hard at ~6.5 (Mohs), is transparent to translucent, mostly colorless, has a white streak, is quite brittle and belongs to the Isometric Crystal System.  Unfortunately, these nice transparent masses rarely form cubic crystals.  In addition, most pollucite is massive and appears as a nondescript off white mineral of unknown origin.

Most of the world’s known reserves, perhaps two thirds, of cesium is in the pollucite found at the Tanco Mine, a tantalum-lithium-cesium, zoned pegmatite in the Lac-du-Bonnet area, northwest of Winnipeg, Manitoba.  Geologists believe reserves of cesium via pollucite from Tanco will last for thousands of years.  My Tanco specimen is massive, dirty gray and nondescript, similar if not identical to other specimens described from the mine.

Massive pollucite from the Tanco Mine.  Width of photo ~3.7 cm.

My second specimen, although also nondescript, is a much cleaner white in color and contains large areas of gemmy and clear pollucite intermixed with the “massive” material. It was collected from a pegmatite known as Uncle Toms’s Mountain near Greenwood, Maine (see Morrill, 1958).

A mixture of massive and more crystalline pollucite collected from Uncle Tom's Mountain. Width of photo ~4.0 cm.

The “nicest looking” specimen of pollucite was collected from Shigar Valley, Skardu District, Balistan, Giggit-Balistan, Pakistan, an area of numerous granite pegmatites and some alpine-type deposits (of which I know little).  This specimen is a “typical-looking” zeolite.  

A very crystalline pollucite from Pakistan.  Typical-looking zeolite.  The arrow points to a possible cubic crystal, otherwise the specimen is a mass of non-crystals. Width of photo ~3.4 cm.

 

Does this story have a moral?  Probably not but I learned an awfully lot about Bunsen burners and their contributions to the discovery and understanding of many elements.  I also tucked away the information that cesium formate is a very expensive drilling mud.  As for the atomic clock, that is way off my radar screen.

 

 

learning curve.