WHAT IS THE WORLD IS TUNGUSITE?

 

BRING ON THE CAKE SINCE EVERY CAKE HAS A STORY TO TELL!

Many times, during my career as a rockhound, I have been visiting a rock and mineral show/shop and have noticed a lonely looking mineral that “lookers” seem to push to the side.  In my mind these specimens deserved a more solid observation.  Usually the specimens were broken, grossly misidentified, glued in ugly cardboard boxes, etc.  However, if you love minerals, minerals will love you back---and there it is, a mineral in a nice perky box that needed a home.

But actually, a mineral hunter like me also needs a sense of humor and “lots of luck.”  Sometimes it works and you get a real jewel. Sometimes you need to make it a jewel, like the mineral tungusite.  When I noticed it at the show the first think that struck me was, “nice new perky box. That is worth a buck, and what in the world is tungusite?”  My curiosity was getting the best of me, so I took it home and promptly forgot about it until this week—and then I thought “what in the world is tungusite?” Turns out it is a hydrous silicate of calcium and iron [Ca₄Fe₂Si₆O₁₅(OH)₆] first described from the Lower Tunguska River, Eastern Siberia Region, Russia, and not found in many other places (Canada, Iceland, India, Portugal, Siberia, Ukraine, and a couple of localities in the Los Angeles area). It does not seem common at any locality, even the Type in Siberia. In all places tungusite forms as a crust in vugs and cavities during the late stages and lower temperatures of hydrothermally altered basalts.  At the TL many of the associated minerals are zeolites but tungusite’s special close friend is gyrolite, a mineral once considered a zeolite.

Platy aggregates of tungusite showing the radial-fibrous structure. Width FOV top ~1.8 cm.

Stacks of laminae clearly seen.  Width FOV ~1.1 cm.

At first glance tungusite appears to have a metallic sheen.  With closer examination one observes the mineral is composed of thin, flexible, radially fibrous plates or laminae that are translucent or transparent. These plates are quite soft at ~ 2.0 (Mohs) and were originally described as yellow green to grass-green in color.  However, as the amount of iron in tungusite varies considerably from almost nothing (white tungusite) to a very dark green, depending on the amount of iron—the more iron the darker the color. A lighter green is only seen when thin sheets are separated from the mass along good cleavage planes. In addition, in close observation, the luster is more pearly than metallic.

As I noted, tungusite often occurs with gyrolite, a sodium calcium hydrate [NaCa₁₆Si₂₃AlO₆₀(OH)₈-14H2O] that is often mistaken for a zeolite as it is associated with zeolites in vesicles, fractures, and vugs of hydrothermally altered basalts, or in skarn-like deposits. It is found in basalts around the world as light colored, often white or brown or green, platy or lamellar masses of crystals that have a vitreous or subvitreous luster.  These masses are soft at ~2.5 (Mohs) and brittle. The mineral is the namesake of the Gyrolite Group where cations vary: if iron is added (tungusite), strontium (cairncrossite), or manganese (orlymanite),

Gyrolite collected from the basalts at Lonovala Quarry, Pune District, Pune Divison, Maharashtra, India. Poona, India.  Width of specimen ~4.5 cm.

Photomicrograph of a section from specimen above showing thin laminae producing a lamellar structure.  Width FOV ~1.0 cm. 

Digging a little deeper, actually much deeper, into a very complicated (at least with my knowledge) paper by Ferraris and others (1995), researchers found that the chemical variability in tungusite is due to microscopic intergrowths of gyrolite between lamellae and also defined a substitutional solid solution between the two minerals.  They also noted that “white tungusite” may be iron-bearing gyrolite, or iron-poor tungusite.  Wow, I thought the recent paleontological arguments about the number of species in Tyrannosaurus rex (Carr and others, 2022)) was complicated but iron poor tungusite vs. iron-bearing gyrolite “takes the cake”! 

Thanks to Ragul Ebinezarm.
 

REFERENCES CITED

 

Carr, T.D., Napoli, J.G., Brusatte, S.L. et al. Insufficient Evidence for Multiple Species of Tyrannosaurus in the Latest Cretaceous of North America: A Comment on “The Tyrant Lizard King, Queen and Emperor: Multiple Lines of Morphological and Stratigraphic Evidence Support Subtle Evolution and Probable Speciation Within the North American Genus Tyrannosaurus”. Evol Biol (2022). https://doi.org/10.1007/s11692-022-09573-1

Ferraris, G., Pavese, A., Soboleva, S.V., 1995, Tungusite: new data, relationship with gyrolite and structural model: Mineralogical Magazine, no. 59.