F is for Fort Wayne Fluorite

When cubic is orthorhombic

Non-cubic fluorite from Ft. Wayne, Indiana. The square insets on the lower part of the crystal at left are also crystal faces, not cleavages. These two were collected by Stan Anderson.

Fluorite, calcium fluoride, crystallizes in the isometric (“same measure”) crystal system, meaning its three crystal axes are all of equal lengths and are all perpendicular to each other. One of the common crystal forms that develops in the isometric system is cubes, and the system is often called “cubic.”

Fluorite often forms cubes, but other forms such as octahedrons, dodecahedrons, and variations of them also form, and all follow the rules of the isometric system, three equal, perpendicular axes.

In the late 1960s and early 1970s an apparently unique find of fluorite crystals came from a stone quarry near Ft. Wayne, Indiana, USA. The crystals were large, remarkably sharp, often water-clear and tinted a light honey-brown to pale golden yellow shade. As far as I know, such fluorite crystals have not been found there since, and their characteristics make them quite distinctive and recognizable.

Another crystal, showing both the sharpness and non-cubic nature of these crystals. At right, unretouched photomicrograph to show the remarkably sharp edges and corner.

Many of the Ft. Wayne fluorite crystals have another characteristic: They are NOT cubes, even though their faces are perpendicular to each other, nor are their edge measurements equal, as they should be in cubes. How can this be?

The photos at the top show the measurements of the most prominent crystals, and it’s evident that they have proportions more like shoe boxes than cubes. Three perpendicular but unequal crystal axes define the orthorhombic crystal system, a distinct system in crystallography. But fluorite can’t be orthorhombic; the molecular structure won’t allow it.

It’s not unusual for crystals to display unequal growth patterns, resulting in skewed or otherwise anomalous shapes, inconsistent with their underlying crystallography. But I think for these exceptionally well-formed, sharp fluorite crystals, we’d expect sharp, well-formed cubes.

Specimens given to me by Susan Vuke.

The easy, cop-out explanation is that for some reason, the crystal faces grew at different rates. That’s simple to say, but harder (for me at least) to understand. Conceivably there could have been some inconsistency in the initial molecular arrangement of the start-up crystal that predisposed the faces to grow at different rates, but for me that’s challenging to reconcile with the exceptional sharpness and unusual clarity of the crystals.

Even if there is some impurity or defect in the crystals giving them the distinctive yellow-brown color, it’s challenging to imagine how that would affect growth rates so much that one face ends up almost twice as wide as another in the same crystal – the definition of isometric is that the crystal is identical in all directions, right down to the molecular level.

from Cave-in-Rock, Illinois, not Ft. Wayne.

Such impurities and defects, together with inconsistencies in the fluid composition, temperature, pressure, etc., most assuredly DO affect crystal growth, so that sometimes “smooth” faces of a crystal may show features reflecting variations in growth, like those above. It’s easy for me to understand variability in the surface growth of one face, but it is much harder for me to understand how two almost pristine-appearing faces of the same crystal can grow at such different rates.

If anyone has an answer, or an idea, feel free to chime in in the comments.

The quarry near Ft. Wayne is in limestones of the Silurian Salina Group, the same resistant rocks that hold up Niagara Falls and form Wisconsin’s Door Peninsula in Lake Michigan and the Bruce Peninsula in Ontario’s Lake Huron. The famous fluorite and rare pyrite are probably related to distant migration of mineral-rich brines from deep basin sources such as the Illinois Basin to the west and the Michigan Basin to the north, onto the flank of the Cincinnati Arch.

The fluorite seems to be quite limited given the short-lived finds of the early 1970s and late 1960s, but it is an active quarry, so who knows how much went into the aggregate piles.

Quarry near Ft. Wayne. Photo Credit