Scapolite

It’s NOT hexagonal

Fig. 1

Spoiler alert (or warning), detailed crystallography ahead. It’s long, and you will be forgiven if you skip this one (or all of them, but then you won’t graduate 😊). Modified from a Facebook post during Covid quarantine.

Meionite, the calcium end member of the scapolite group of aluminosilicates, is tetragonal, meaning it has a 4-fold symmetry. A cross-section ought to be (and often is, to the point that it’s sometimes almost diagnostic) square, or a modified square. So why is the cross section of the crystal at right in the photos above, looking down the long c axis, a near-perfect hexagon?

I know for sure that this is meionite, Ca₄Al₆Si₆O₂₄CO₃, because it was analyzed by x-ray diffraction and Raman spectroscopy, so it must be tetragonal. I’m also quite sure this is a nearly perpendicular cross section, so it’s not some oblique cut giving funny angles. I’m equally certain that it is not scapolite replacing some other mineral that was hexagonal.

Scapolite is also notorious for having unequally developed faces – so its true symmetry isn’t necessarily reflected in the way the crystals actually grow. I think that’s what has happened here. The “hexagon” cross section shows the prism faces of the crystal but only one pair directly expresses the tetragonal symmetry, while the other four are second- or third- order prisms that are incompletely developed and lack the faces that would define 4-fold symmetry – there should be 8 faces, but two failed to develop.

Fig. 2. Specimen at right attributed to La Panchita Mine, Ayoquezco de Aldama, Oaxaca, Mexico.

The three-dimensional sketch above shows an idealized tetragonal prism, together with an actual scapolite crystal. I’ve highlighted the edges of the top face (the one labeled (001) in the drawing) and indicated the two other crystal axes in yellow, which show the 4-fold symmetry as well as the irregularities in it in the natural crystal.

Fig. 3

The cartoons above look down on the crystals from the top, showing the way the vertical faces (the sides of the drawings) in the 3-d drawing might have developed.

In A, the 4-fold symmetry is still quite evident as the original square set (labeled (100) and (010)) has been modified with a symmetrical set (110) that knocks off each corner.

In B, the symmetry is less apparent because the second prism (120) has developed pairs of faces unequally. This (and variations of it) is actually a pretty common shape for scapolite crystal cross sections.

In C, the unequal development is more pronounced, but if you can identify the original four faces (100 and 010) even though they are smaller, you can still see the 4-fold symmetry, but it is quite confused by the extra prisms (120 and 320) failing to develop all four faces as they should have. It’s not that unusual for things like this to happen.

In D, because those extra prisms, without the complete set of 4 faces each, have grown so large, the original (010) faces on the left and right have been eliminated. So the apparent symmetry is no longer 4-fold (tetragonal), but it’s STILL not actually hexagonal!!

Fig. 4

The rock is from an outcrop along the road from Pipestone to Delmoe Lake, east of Butte, Montana USA, in a narrow zone of rocks that have been caught between and cooked by two separate igneous plutons of the Boulder Batholith. (More info in my paper, “Geology and mineralogy of the southeastern margin of the Butte Pluton near Pipestone, Montana,” in Montana Bureau of Mines & Geology Open-File Report 699, 2017.)

Fig. 5

Meionite at that locality typically fluoresces magenta to red, and in places may be completely included in vesuvianite, as shown in the photos above.

Fig. 6

The scapolite is also closely associated with the zeolite stilbite. Analysis could not discriminate between stilbite-Ca and stilbite-Na, but this is almost certainly the calcium member, NaCa₄(Si₂₇Al₉)O₇₂ · 28H₂O because all the other minerals in the skarn there are derived from metamorphosed carbonates (limestones and dolomites).

Scapolite was named in 1800 by José Bonifácio de Andrada e Silva from Greek ζκαποζ, zkapoz, a shaft, in allusion to its typical long prismatic crystals. Meionite was named in 1801 by Rene Just Haüy from μειον, meion, Greek for less or decrease, in reference to the less acute pyramidal form as compared to vesuvianite.