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.