BABINGTONITE: MA STATE MINERAL

I was rummaging around some web sites the other day and reading about various minerals that some states had designated as their official “State Mineral.”  Most were quite familiar such as rhodochrosite (Colorado), gold (Alaska) and copper (Arizona) among others.  However, one particular mineral that caught my eye was babingtonite, the official mineral of Massachusetts.  I searched the back recesses of my mind and sort of remembered that it was a dark colored mineral, a calcium iron magnesium silicate somehow associated with zeolites.  I also knew that a small specimen was somewhere in my collection---so I dug it out; however, it was not from Massachusetts but was collected at the Iron Cap Mine in Arizona.  I also then made a decision to hunt for a New England specimen during my February travel to the Tucson Shows.

I enjoy hunting for unique minerals in all sorts of small mineral venues and in dusty trays and so was rewarded in Tucson with a nice glittery Massachusetts specimen with hundreds of small babingtonite crystals.  Another serendipitous moment!

Babingtonite [Ca₂(Fe,Mn)FeSi₅O₁₄(OH)] crystals have a very dark green to black color and a vitreous luster.  It is fairly hard at 5.5-6.0 (Mohs) and has stubby to prismatic and striated crystals.  They are opaque and actually quite impressive.  They also are difficult to photograph, at least to me.

Babingtonite is in a solid solution series with manganbabingtonite where some of the iron is replaced with manganese [Ca₂(Mn,Fe)FeSi₅O₁₄(OH)].  It is probably impossible for me to tell the difference between the two minerals.

Although babingtonite sometimes occurs in low temperature hydrothermal veins found in granite pegmatites, it is most common in mafic volcanic basalt where it is found in vugs and often associated with various zeolite minerals, quartz, calcite and prehnite. 

Tiny black crystals of babingtonite on a matrix.  Length of specimen ~4 cm.  The crystal faces reflect light.  Specimen collected from Blueberry Hill Quarry, Massachusetts.

Parts of Massachusetts have several bedrock quarries where “trap rock” is quarried for dimension stone or road aggregate.  Trap rock is just a common name that refers to extrusive volcanic rocks such as basalt or a fine grained intrusive rock commonly found in sills and dikes (diabase).  In fact, much of New England displays erosion-resistant ridges representing Triassic intrusive or extrusive rocks trending across the countryside.  Many of the ridge quarries contain rocks with vugs of all sizes.  These openings in turn contain a wide suite of zeolite minerals along with other minerals such as prehnite and babingtonite and are a collecting source for New England rockhounds.  These basalts and diabases are associated with the breakup of supercontinent Pangaea into the proto-continents North America and Europe.  As Pangaea split, a series of valleys formed, basalt squeezed out, and intrusive sills and dikes formed.  

Photomicrograph of section of specimen above.  The individual crystals are 1 mm. and smaller.  These tiny black crystals are difficult to photograph.

My purchased specimen came from the Blueberry Mountain Quarry near Woburn, Massachusetts where MinDat lists 32 valid minerals. Beside the nice crystals of babingtonite, the specimen contains extremely small strands of “byssolite,”something not listed by MinDat as being present.  “Byssolite” is not a valid mineral but a variety of the Amphibole Supergroup and may be hair-like fibers of several minerals.

Small strands of “byssolite” with the largest ~ 1mm in length.  Same specimen as above.

The initial specimen of babingtonite in my collection, acquired several years ago, came from the Iron Cap Mine, Landsman Camp, Aravaipa District, Graham County, Arizona. I later found out that the identification was incorrect and the crystals were manganbabingtonite.  Of course I cannot extinguish between the two minerals based on external physical characteristics!

Crystals of manganbabingtonite with acicular crystals of johannsenite.  Collected at Iron Cap Mine.  Length of left largest crystal ~3 mm.

Individual crystal of manganbabingtonite from the Iron Camp Mine.  Length about 2.5 mm.

Another mineral of interest in the specimen is johannsenite, a somewhat uncommon calcium manganese silicate [CaMnSi₂O₆], sometimes containing iron, which is the dominant pyroxene at the Iron Cap Mine (Anthony, 1995). The physical properties of johannsenite vary: color ranges from brown to black to gray to green to light blue to yellow to violet and others; it is translucent to transparent; the habit is massive to acicular needles to radiating aggregates to splintery; the luster varies from greasy to vitreous and the hardness is 6 (Mohs) although the acicular needle masses break apart easily. It forms in contact metamorphic zones—as does the babingtonite. Johannsenite in my specimen is composed of massive green prismatic crystals or cleavage fragments (angles of 87⁰ and 93⁰ typical of pyroxenes).

A second specimen of johannsenite from the Iron Cap has very dark green patches of acicular crystals.

Johannsenite is in solid solution with hedenbergite when the iron completely replaces the manganese [CaFeSi₂O₆] and with diopside as magnesium replaces the manganese [CaMgSi2O6].  In a process that somewhat confuses me as to process, johannsenite alters to pink rhodonite (see Livi and Verblen, 1992, for a detailed report on this process.)

Green patchy acicular crystals of green johannsenite (J) with cream-colored masses of tiny acicular crystals of something, perhaps ??nekoite (?)[Ca₃Si₆O₁₅-7H₂O] and metallic galena (G).

The Iron Cap Mine is a former surface and underground Pb-Zn-Ag-Cu-Au-Fluorspar mine where the major ores were sphalerite (zinc) and galena (lead).  Mineralization is found in vein deposits hosted in the Horquilla Formation (Pennsylvanian) and the Pinkard Formation (Cretaceous). Some ore veins occur in faults between formations while others are found wholly in the limestone beds. The mine area also includes numerous intrusive veins of Cretaceous and Tertiary age cutting across Paleozoic rocks (Simons, 1964).

Anthony and others (1995) noted the Landsman claims contain manganbabingtonite in veins containing sphalerite and galena in a contact metamorphic zone developed in limestone and shale.  Therefore, the occurrence is different than the Massachusetts basalt vugs.

I've always loved life, I've never known what's ahead.  I love not knowing what might be round the corner.  I love serendipity.

                                 Twiggy

REFERENCES CITED

Anthony, J.W., S.A. Williams, R.A. Bideaux, R.W. Grant, 1995, Mineralogy of Arizona (Third Edition): The University of Arizona Press, Tucson.

Livi, K.J.T. and D.R. Veblen, 1992, An analytical electron microscopy study of pyroxene-to-pyroxenoid reactions: American Mineralogist, Volume 77.

Simons, F.S., 1964, Geology of the Klondyke Quadrangle, Graham and Pinal Counties, Arizona: United States Geological Survey Professional Paper 461.