Unusual Mica Triplets from West Pierrepont, NY
By Steve Chamberlain
Fellow of the NYS Academy of Mineralogy
Mica minerals are monoclinic, 2/m, which means that their a axis and b axis are perpendicular to each other, but their c axis is tilted slightly from being perpendicular to the plane of the a and b axes. That's why an elongated "hexagonal prism" of mica won't stand straight up on the table; it's not really hexagonal. The symmetry elements in a typical six-sided mica plate are a mirror plane containing the a axis and a two-fold axis of rotation coincident with the b axis. Those are the only two symmetry elements: a mirror and a perpendicular two-fold axis of rotation.
Figure 1. Pseudohexagonal crystal of mica showing the two symmetry elements, a mirror plane containing the a axis and a two-fold axis of rotation coincident with the b axis. Note that the left and right faces together form a b-pinacoid and the top two and bottom two faces together form a monoclinic prism. The top and bottom angles are the same as are the left two and right two angles; however, none of them are exactly 120° as would be expected if mica were actually hexagonal.
According to the mineralogy textbooks and to Goldschmidt's Atlas der Krystallformen mica twins are typically contact twins with c(001) being the contact plane. Basically, they are not very interesting as twins go.
Imagine my surprise when in late spring a friend showed me a phlogopite crystal from a new trench he had begun digging at the familiar locality for tremolite and uvite near West Pierrepont in St. Lawrence County, NY, that appeared to be a three-fold twin on the cleavage termination (c-face). He collected a large number of these. Mike Walter also collected some very good examples, and I managed to collect a few. Initially, I thought I was looking at a growth distortion since there is no three-fold axis in the diagram above. Finally, Mike Walter suggested these were actually twins and were related to the star mica specimens that were also twins.
Figure 2. A 3.8-cm phlogopite triplet from West Pierrepont. This specimen was collected and photographed by Mike Walter. The longest of the three twins sits on the mirror plane (a axis). The orientation is looking down the c axis directly at the plane of cleavage, c (001).
Figure 3. This idealized drawing of a phlogopite triplet shows the prominent three twin members at the termination and the gradual expansion toward the "hexagonal" girdle at the center of the crystal. Most actual crystals do not look very hexagonal in the middle.
Now I had sitting on my desk an array of phlogopite crystals I basically thought were impossible—three-fold twins in a monoclinic mineral without a three-fold axis. I started my research with the star micas that have been found at several places in Brazil and in Pakistan. Figure 4 shows an iconic image of a Brazilian star mica twin of muscovite.
Figure 4. Star muscovite from Minas Gerais, Brazil. Photograph by Rob Lavinsky. Taken from the website <mindat.org>.
I have to admit that the above specimen looks awfully much like a hexling, a six-member twin like the cyclic twins of rutile. As I continued to look for crystallographic literature that explained how these made any sense, I came across an amazing 1964 paper by R. W. Rex on phase equilibria at low temperatures. He was using authigenic kaolinite and mica that formed in a sandstone as his model system. The paper was published in the 13th National Conference on Clays and Clay Minerals. (Thank goodness for Google Scholar!). This paper shows a number of pictures of microscopic mica three-fold and six-fold twins, which Rex referred to as triplets, and by extension, hexlets. Two of them are shown below in modified form.
Figure 4. Two arms of a muscovite triplet lying above another mica crystal. This is a transmission electron micrograph made from a replica. From Rex (1964).
Figure 5. A muscovite hexlet. This is a transmission electron micrograph of a replica. From Rex (1964).
In his paper, Rex does not consider these kinds of triplet and hexlet mica twins at all unusual, suggesting that microscopic authigenic twins in sedimentary environments are well known to some specialties of geology. Maybe so, but they were not familiar to this specimen mineralogist and are still very puzzling.
As I continued my literature search, I found two interesting papers that deal with the issue of three-fold or six-fold twins in monoclinic minerals. One is a 1964 paper on growth spirals on phlogopite crystals by Ichiro Sunagawa. This paper suggests the possible existence of three-fold and six-fold twins in monoclinic micas. The second is a 2000 paper by Massimo Nespolo and colleagues that explains how merohedry produces triplets and hexlets in basic mica polytypes. This paper uses a branch of mathematics called group theory to treat the crystallographic structural issues involved, and I must admit I don't understand any of it.
So I have come to accept that these are actual triplet twins of phlogopite, even though I don't thoroughly understand how they fit into the crystallographic picture. By carefully examining the dozens and dozens of specimens I now have, I do understand something about how they formed. A few of the triplets are doubly terminated and have a "Y" pattern on each end. If you orient one end as shown in figure 3 and then rotate it 180° around the b axis as shown in the diagram, the "Y" pattern on the bottom rotates into the same position on the top. This indicates that there is an untwined zone of phlogopite in the middle of the crystal. The top triplet is oriented to the "hexagonal" plate of the untwined central zone in the same way as the bottom triplet and they are related by two-fold rotation. A few of the triplets seem to be continuous from top to bottom so that the same kind of rotation reverses the "Y" pattern. Most of my specimens, however, show a triplet on one end and a cleaved, untwined plate of mica on the other. Figure 6 explains the interrelations of these three patterns. Remember that mica cleaves very easily and these are about 1.2 billion years old.
Figure 6. Schematic diagram showing how different cleavage planes produce three different kinds of triplets. The sketch is a tapered phlogopite xl viewed along the plane of cleavage (striations). If a triplet cleaves as indicated in A, then it will be a continuous single triplet from top to bottom. If it cleaves as indicated in B, then one end will be a triplet and the other end will be an untwined plate of phlogopite. If it cleaves as shown in C, then there will be triplets on both ends, but they will be related by rotation around the b axis.
These unusual triplets were found in two nearly parallel seams, both partially filled with pink calcite. In one seam, white albite and green tremolite formed the matrix, but in the other it was mainly green tremolite. Many of them had already weathered free of the calcite and cleaved from the wall and were found loose in the soil. Others were etched from solid sections of the seam yielding undamaged matrix specimens. Adjacent to these two seams was a third comprised mainly of marialite, partially altered to microcline and covered in places with beautiful transparent colorless crystals of heulandite. Mike Walter <geologicdesires.com> has a nice selection of these unusual triplets available.
Nespolo, M., Ferraris, G., and Takeda, H. (2000) Twins and allotwins of basic mica polytypes: theoretical derivation and identification in the reciprocal space. Acta Crystallographica A56:132-148.
Rex, R. W. (1964) Authigenic kaolinite and mica as evidence for phase equilibria at low temperatures. Clays and Clay Minerals25:95-104.
Sunagawa, I. (1964) Growth spirals on phlogopite crystals. American Mineralogist 49:1427-1434.