RARE BARIUM SILICATES:SANBORNITE AND MACDONALDITE

Most rockhounds are familiar with the element barium although they have never seen the element in its natural state—it is never found in nature as a free element.  We know the element due to its combination with the sulfate ion, SO₄, to produce the mineral barite or baryte—BaSO₄, or to the lesser known barium carbonate, BaCO₃, the mineral witherite. Other than those two minerals many of us would be hard pressed to talk about other minerals containing barium as it is mostly an accessory element, or minerals with barium as the major cation are rare.

I certainly knew very little about barium minerals until I found an article by Dunning and others (2018) describing the distribution of barium silicate minerals from Baja California, Mexico, north to Alaska.  It is a comprehensive, three-part series and may be found at: www.baymin.org/papers.

Barium silicates are rare minerals and many of the described sites only contain one or two different species, yet these total sites contain 44 different minerals.  Even today there are new barium silicate minerals being discovered and described.  About the only minerals on the list that I recognize are joaquinite-Ce [NaBa₂Ce₂FeTi₂Si₈O₂₆(OH)-H₂O] and benitoite [BaTiS₃O₉] collected from the famous Dallas Gem Mine in the San Benito Mountains of California.  The blue gem benitoite is the State Gem of California.  Both minerals form in fracture fillings correlated with subduction zone rocks (serpentinite, greenstone, blueschist) associated with converging plate boundaries, mostly at high pressures and low temperatures.

After the Dallas Gem Mine the best-known locality for barium silicates is Big Creek-Rush Creek Mining District in Fresno, County. I could not locate much information about the mining history other than the mines produced barium.  To rockhounds, the interesting aspects of the District are the mines that have produced the type locality of 16 rare barium silicates plus they are the home of at least 13 other rare barium silicates.  By my count that is 29 different rare barium silicate minerals at this single locality. In examining mineral photos from Big Creek-Rush Creek on MinDat it is interesting to note that many of these barium silicates only have three to four photos displayed on the web site.  Although the number of photos on MinDat is not a solid indicator of mineral abundance, it certainly gives the reader a decent indicator of rarity to abundance. 

 

From a Denver Show many years ago I picked up a perky box with a specimen containing the barium silicates macdonaldite and sanbornite collected from the Big Creek-Rush Creek District in California. I purchased it since the District is the Type Locality of macdonaldite (the wonders of cell phones to examine MinDat when looking at minerals).  It certainly was worth the two bucks I paid, and there are probably additional barium silicates in the specimen.  

Macdonaldite is a barium calcium hydrated silicate [BaCa₄Si₁₆O36(OH)₂-10H₂O] that has a silky luster (may appear at times to be vitreous). It is soft at ~3,5-4.0 (Mohs), usually white to perhaps colorless, and is transparent to translucent.  MacDonaldite usually appears as acicular crystals or fibers arranged as a white radial group.  At Big Creek-Rush Creek macdonaldite appears as veins and fracture coatings in a sanbornite and quartz bearing metamorphic rock, the result of contact metamorphism of sedimentary rocks by a Late Cretaceous granodiorite pluton (Dunning and others, 2018).  Macdonaldite is a low temperature and late appearing mineral and may, in most cases, be an alteration product of sanbornite (Dunning and others, 2018).

 

A large radial group of macdonaldite crystals and fibers located on sanbornite.  Width FOV ~9 mm.

A small group of macdonaldite acicular crystals (M) less than 1mm in width. Numerous other minerals, perhaps barium silicates.

I presume the brown to brown-yellow mineral surrounding macdonaldite (M) (~1 mm) is a barium silicate (maybe even two or more). Perhaps it is verplanckite?

 

Could the orange be muirite?

Sanbornite, a barium silicate [Ba₂(Si₄O₁₀], is colorless to white or perhaps pale green, but forms platy sheets with good cleavage.  The sheets are often iridescent and have a vitreous to pearly sheen.  It is transparent to translucent with a hardness of 5.0 (Mohs). Sanbornite occurs in veins of metamorphic quartzites (previously sandstone) and hornfels (previously shale/siltstone; low pressure; moderate to low temperature ~400-600 C) (Dunning and others, 2018) heated by igneous plutons, and almost always occurs with quartz.

 

A stack of horizontal sanbornite sheets with the arrow parallel to the sheets. The white coating is probably macdonaldite. Width FOV ~1.4 cm.

Looking at the top layer of the stack shown above.  Note pearly to vitreous luster and the iridescence. Width FOV ~1.4 cm.

I have been trying to compare the barium silicates with the calc-silicates; however, that may be above my pay grade.  The calc-silicates [Ca₅(SiO₄)₂(CO₃)] usually form in high-temperature, contact metamorphic zones where a granitic-dioritic magma (high magnesium, silicon, aluminum, and iron content) intrudes into cooler (lower temperature) limestone or other carbonate rocks.  The invasive hydrothermal fluids alter the limestone into other minerals such as iron oxides, calc-silicates (wollastonite, diopside), andradite and grossularite garnets, epidote and perhaps ore minerals. These altered carbonate deposits are termed skarns. 

The barium silicates also form in low to high temperature, low pressure contact metamorphic zones.  My question involves the original source of the barium—where did it originate?  I did find out from Dunning and others (2018) that mixtures of BaCO₃ plus SiO₂ yielded BaSi₂O₅ plus CO₂ according to the equation: BaCO₃+ 2SiO₂→ BaSi₂O₅+ CO₂↑. This equation shows witherite and silica produced sanbornite plus CO₂ vapor at a temperature range from 440 C to 600 C.  Other stoichiometric mixtures of silica + baryte failed to react at temperatures up to 750 C. Virtually no baryte broke down at these temperatures. These results would appear to eliminate baryte as a direct source of the barium for the formation of sanbornite during metamorphism.  The reaction of barium carbonate and silica to form sanbornite and carbon dioxide is analogous to the well-known reaction of calcite and silica forming wollastonite and carbon dioxide. So, does the formation of all barium silicates require the presence of barium carbonate? 

If I understand Dunning and others (2018), the presence of barium carbonate was required for the formation of most barium silicates although a few minerals are the result of secondary alteration of sanbornite or gillespite. 
 

1.    Sedimentary baryte, or witherite, precipitated locally as part of an abyssal sedimentary sequence in a possibly continuous narrow basin off the west coast of Mexico and California within the late Paleozoic to Early Jurassic time period.

2.    Diagenetic activity then dissolved the baryte and re-precipitated the barium as witherite.

3.    Burial of the sediments to depths was followed by one or more periods of severe deformation, probably caused by subduction along a continental margin and thermal metamorphism attributable to the intrusion of large granitic plutons. The temperature of metamorphism was between 400 C and 600 C.

4.    Metamorphic action on residual sediments rich in barium (the witherite) and other elements is the major reason for the majority of barium silicate occurrences.

5.    The majority of barium silicates identified in this study are of primary origin.

6.    The secondary alteration of sanbornite during low temperature hydrothermal activity has produced macdonaldite.

In my case, this is  one of life’s persistent questions that has been ferreted out by locating the tremendous article by Dunning and other (2018).

REFERENCES CITED

G.E. Dunning, R.E. Walstrom, and W. Lechner, 2018, Barium Silicate Mineralogy of the Western Margin, North American Continent, Part 1: Geology, Origin, Paragenesis and Mineral Distribution from Baja California Norte, Mexico, Western Canada and Alaska, USA: Baymin Journal, Vol 19, No. 5.

FLUORAPATITE, BERTRANITE AND MUHAMMAD ALI

I know little about the geology of Maine and in past excursions was more interested in the fantastic opportunities to sample seafood and explore the home store of L.L. Bean.  Early in my career I spent a week camping along the shoreline while pulling a large pop-up camper trailer.  Sure, I pounded on the rocks but also spent more time stopping on the roadside so we could gorge ourselves on wild berries and hitting the fishing docks in late afternoon to purchase lobsters and mussels right off the boats.  Wow, what a treat.  I also took a very early morning hike to the top of Cadillac Mountain on Mt. Desert Island to see the sunrise since that spot receives the first sunshine of the day in the lower 48.

Later, when I was on the undergraduate research speaking circuit, I presented at Bates College, a beautiful private liberal arts college in Lewiston, Maine.  A “field trip” one evening took the group over to the L.L. Bean home store in Freeport.  In those “olden days” most of us were not living close to a Bean outlet so the home store was just like Christmas.

Lewiston is also well known for a singular event that took place on May 25^th, 1965, when Muhammad Ali knocked out Sonny Liston in the first round of a Heavyweight Championship fight.  After the fight, the 23-year-old Ali called the punch that dropped Liston his secret: "It was a phantom punch. It was lightning and thunder — fast as lightning and booming as thunder from the heavens,"


Now that I have more time I am trying to learn, something in detail, about the geology of New England.  When I taught Stratigraphy, I told my students that we were lucky to live in the Plains’ states since you could actually see rock outcrops.  In New England, the rocks are all covered by vegetation!


The other day I was thumbing through a copy of the Nov/Dec volume of Rocks and Minerals looking for an article containing information on the gem zoisite variety known as tanzanite.  After reading about that gem I stumbled on an article entitled The Emmons Pegmatite: Greenwood, Oxford County, Maine. Something popped up in my mind that said, “you have a specimen from there.”  So, I took a peek in the drawer and there was a small plastic cube box with a specimen collected by David Shannon in August of 1997 or 1999 (ink smeared).


The Emmons Pegmatite was well described by Falster and others (2019) and I urge readers to examine that issue of Rocks and Minerals where I retrieved the following information.


The Emmons Quarry is found on Uncle Tom Mountain in Oxford County, Maine, and was first mined for feldspar in the 1930s; however, that venture was short-lived.  Mineral enthusiasts then started extracting beryl and collected about 5000 carats of gem morganite (pink beryl). Specimen collecting has continued to the present and is especially active with the move of Alexander Falster and William Simmons (and their laboratory) from New Orleans to the Maine Mineral and Gem Museum in Bethel. The Emmons is known for its many phosphate minerals and is Maine’s most species rich pegmatite (109 valid minerals according to MinDat).


The Emmons is part of the New England Appalachian Mountains and the entire New England area has a complex history.  I always had a great deal of respect for the geologists deciphering the history of these rocks, especially with the aforementioned vegetation!  I have lived through an exciting time in the history of geology where “continental drift, eugeosynclines and miogeosynclines were strange bedfellows in my 1960s era Historical Geology class. Today, 60 years later, the entire concept of plate tectonics has evolved into academic dogma.  In my class the text “talked” about Paleozoic mountain building events in the eastern U.S. (todays geography); however, these tectonic pulses, named Taconic (Ordovician), Acadian (Devonian), and Alleghenian (Permian), were distinct events and no one really understood their “cause” or relationship  We just studied uplifts and erosion.  Today we know that shifting plates were colliding with one another, some were subducted with resulting volcanism and intrusions and metamorphism, and microcontinents were often caught in the middle and were accreted to larger continents. All of this action was continuous during the Paleozoic resulting in the supercontinent termed Pangea. Rocks of the Emmons Pegmatite were originally marine sediments deposited in a deep-water marine basin during the Ordovician-early Devonian and were later subjected to deformation and metamorphism during Devonian to Permian tectonism.  For a discussion on the breakup of the supercontinent see Posting October 21, 2019.


My thumbnail specimen that I pulled from the back of the drawer is amazingly rich with albite and other feldspars, manganese dendrites, quartz, several unknowns, bertrandite [Be₄(Si₂O₇)(OH)₂], and fluorapatite [Ca₅(PO₄)₃F]—and probably others.  I purchased it for the nice purple crystals of fluorapatite and the gemmy clear bertrandite for which the quarry is famous. At the Emmons Quarry Falster and others (2019) noted that both bertrandite and fluorapatite result from the corrosion and alteration of beryl and form in the vacated cavities/vugs. 

Purple fluorapatite (F) and clear gemmy bertrandite (B). Width photo ~7 mm.

Note the tiny, submillimeter, tan prismatic crystals marked with a ?, along with fluorapatite and bertrandite.

I presume the lower arrow points to a number of lilac to clear crystals of fluorapatite (submillimeter).  The upper arrow points to a similar situation of stacked crystal of a tan-orange color.  Some of the black mineral may be manganese oxide. Lots of questions!

More questions!

The scientist is not a person who gives the right answers, he's one who asks the right questions.  Claude Levi-Strauss 

I presume the large clear crystal is fluorapatite due to the striations

A nice mixture in a vug.

In Maine we have a saying that there's no point in speaking unless you can improve on silence.        Edmund Muskie

REFERENCES CITED

Falster, A. U., Simmons, W. B., Webber, K. L., Dallaire, D. A., Nizamoff, J. W., & Sprague, R. A., 2019., The Emmons Pegmatite, Greenwood, Oxford County Maine: Rocks & Minerals, v. 94, no.6).