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