WOLLASTON, WOLLASTONITE, AND A CAMERA LUCIDA

During my early career as a paleontologist I struggled with how to depict small rodent teeth in professional publications.  How does an author present “what the teeth look like” to a professional audience?  This was "a long time ago" before digital photomicrographs, or even before third tube stereomicroscopes allowed for the addition of a film camera.  I tried close-up lenses and bellows but without much success.  Then, a more seasoned paleontologist introduced me to a camera lucida.

 

Artist using a version of a camera lucida.  Paleontologists and other scientists use a camera lucida that attaches to a stereomicroscope tube.

This device, fitted on a stereomicroscope tube, allowed an optical superimposition of the tooth being viewed onto an adjacent piece of paper.  I then was able to view the tooth and at the same time sketch the object on the paper and render an accurate depiction of the fossil.  Wow, this was a major accomplishment.

So, in those early days I depended a great deal upon this unique device.  In fact, I used it extensively until the Scanning Electron Microscope came along and made life a little easier.  In later years, I begin to wonder---who was the person responsible for “inventing” the device?  Was it an artist?  A scientist?  Some other investigator? 

 

It turns out that a chemist-physicist by the name of William Hyde Wollaston (England; 1866-1828) was responsible for the camera lucida.  And, the interesting secondary part of the story is that Wollaston was a world-class scientist (also a practicing physician) and responsible for a number of scientific discoveries.  Perhaps he is best known for discovering the chemical elements palladium (Pd) and rhodium (Rh)---not bad considering there are only 100 or so elements known!  He made a large amount of money by developing and putting into practice a method for purifying platinum and dabbled in trying to understand electricity and batteries and their relationships to electric motors.  He also worked with optics trying to determine the elements present in the sun, and developed a reflecting goniometer (mineralogist use such to measure crystal angles).  And, of course he discovered some specific camera lenses, and the camera lucida in 1812.  Wollaston was an intellectual heavyweight.

After reading an abbreviated bibliography of Wollaston, my mind jumped back to the world of rockhounding and a mineral called wollastonite (CaSiO₃)---certainly it must be named after the famous scientist.  I remembered it being some sort of an interesting mineral that combined calcium (but not much carbonate) and the silicate compound, affectingly termed a calc-silicate by mineralogists.  Other members of that group include such minerals as diopside [CaMg(Si₂O₆)] and grossular (Ca₃Al₂(SiO₄)₃]. The calc-silicates generally are associated with impure metamorphic limestones and dolomites, and seem especially common in skarns—the contact zone around igneous intrusions and carbonates. 

 

Wollastonite, mass of cleaved surfaces. Length 3.2 cm.  

Wollastonite is usually white in color, in fact a bright white, or at times rather colorless (although other colors exist), with a hardness of 5 or slightly less (Mohs).  The colorless variety is transparent while the colored crystals are translucent to slightly opaque.  Much/most of the time it is massive in nature with few observable crystals, and has a cleavage of ~90^o in two directions.  The luster is vitreous to pearly.  I thought that the specimen in my collection was a mass of small crystals; however, upon reading descriptions, and re-observing with a scope, it appears to be a mass of tabular/bladed, splintery, cleaved surfaces. 

I had really never wondered about commercial uses of wollastonite until composing this posting—but I found the answers from an economic geologist quite interesting.  It is quite important in the manufacture of tile and ceramics, plastics, and paint.

The specimen in my collection came from the famous Crestmore Quarry near Riverside, California.  Www.mindat.org  describes the geology as:  Two irregular, very roughly parallel, lenticular bodies of magnesium-rich limestone of Mississippian age were deposited, each approximately 400 feet thick, and whose principal portions … conforms to the trend of the Peninsular Range to the south, and is almost perpendicular to that of certain other metamorphic rocks near Riverside. The lower is the Chino Limestone and the upper is the Sky Blue Limestone. They are separated by gneissic hornfelses, quartz-mica schists and diorite. The beds were metamorphosed and recrystallized into marble during the early Triassic. Several later magmatic intrusions produced a complex suite of contact metamorphic minerals.  In addition, on their website I count something close to 150 different minerals collected from the quarries.  Wow.

I have one other specimen from Crestmore that also is quite interesting-- torbermorite.  I picked this up since I didn’t have the slightest idea what the specimen represented, and part of it was a beautiful sky blue in color!  Who could pass up a two dollar specimen with an interesting sounding name and a nice color?

 

Blue calcite with torbermorite and ellestadite.  Length of specimen is ~2.7 cm.

As it turns out, torbermorite is also a calc-silicate, sort of:  Ca₅Si₆O₁₆(OH)₂-4H₂O.  It has the calcium and the silicate along with the hydroxyl group (OH) and water (H₂O).  On my specimen it turns out that the sky blue section is calcite (CaCO₃) while torbermorite is a white, translucent, soft (2.5 Mohs) bundle of tiny laths.  Although I certainly am not familiar with the mineral, it seems not uncommon in areas of metamorphosed carbonates.

But wait, in examining the specimen under a scope I noticed two different “light colored” minerals, or at least they could be two different minerals as they were slightly different colors.  So I consulted MinDat and “discovered“ a carbon copy photo of my specimen.  The cream-colored second mineral was listed as ellestadite—[Ca₅(SiO₄,PO₄,SO₄)₃(F,OH,Cl)].  Now, this mineral seems way out of my league in understanding the mineralogy.  Best that I can tell it is a type of apatite, or at least closely related to apatite---a calcium sulfate silicate.  It appears in metamorphic carbonates and in the U.S is only found at the Crestmore quarries.

I love life and the interesting roads that are available for travel.