The Uncommon and the Small:
Phosphate Cave Minerals in Rippled Cave
by Bruce Rogers
Published: Fall, 2004

In the natural world it is true that the closer one looks, the more complex things become. This is just as true in the world of caves. These darkened realms have many splendid vistas, lofty chambers, and gargantuan stalactites; however, when one looks closely, one finds more than first meets the eye.

Rippled Cave, located deep within the Mother Lode, indeed has a huge chamber and lofty halls among its attributes. On a smaller scale, one may notice the scalloping of the walls that lends its name to the cave itself. Looking closer, the keen observer will note that delicate filigrees of calcite cave coral march along the arêtes delineating each basin-sized scallop. Still closer, one will note small, apparently smooth, chocolate-brown flowstone sheets tucked away in corners of the chambers. All is not what it appears, though, within these sheets.

The western complex of cave passages ends at the Scallop Room, a large hall with scalloped walls and a large chockstone wedged in its northern extreme. High on the east wall of the Scallop Room is a thin, dark, chocolate-brown sheet of flowstone. The years have not been kind to this area, however, and the press of many climbing boots has fractured the edge of the sheet. Close observation reveals that the brittle flowstone is only an eighth of an inch thick. Under it lies a quarter-inch-thick layer of yellow, plastic material that looks for all the world like thick, pale mustard. In 1976, tiny samples of both these materials were collected for study.

To our amazement, the flowstone was not the expected calcite. The material turned out to be the rather uncommon cave mineral hydroxylapatite - hydrous calcium phosphate. The underlying cream-colored layer, although of vastly different texture and color, was also hydroxylapatite. Both these samples were mounted in a scanning electron microscope (SEM) wherein bundles of electrons are passed over the sample and massaged by black boxes. When the machine was focused into looking at the samples under several tens of thousands times magnification, the resulting photographs were spectacular to say the least.

Apatite is actually a generic name for a small group of phosphate minerals with similar crystalline structure. The name, coined in 1788, comes from the Greek "to deceive" since clear, flawless crystals looked similar to other gemstones. Apatite has hexagonal symmetry - that is, its crystals are six-sided prisms with low six-sided pyramids at each end. While the crystals can be elongated, usually they are a more squatty, barrel shape. Bones and teeth are largely made of one of the apatite minerals, dahllite, a closely related calcium carbonate phosphate. Apatite is used in fertilizers and gemstones, although its marginal hardness, barely less than a knife blade, precludes its extensive use in jewelry. The mineral is usually vitreous and often transparent to translucent. Color shades from green and brown through yellow, blue, violet, and even colorless are known. It is uncommon to rare, but wide-spread throughout the spelean world.

Even under the tremendous magnification of the SEM, the hard, brittle flowstone had little relief on its surface, appearing rather smooth and non descript, as any "normal" flowstone should. A scattering of thin, elongated crystals was found growing on the surface, but by and large, the surface was unspectacular.

A thin transition layer was found under the hard crust. This consisted of an irregular layer of thin crystals intricately meshed together much like jackstraws thrown into a pile. The further one looked from the surface, the denser the mesh became, until it resembled a pile of randomly stacked sheaves of wheat. At the base of the layer, the individual crystals merged into irregular nodular masses and sheets.

The underlying plastic layer, which could be called moonmilk, was fascinating in its variety of forms; and considerable time was spent "touring" about the sample. Viewing under lower magnification showed an irregular, hackly surface. With increasing magnification, however, a wilderness of rounded ridges and valleys became visible. Long, thin, curved filaments that looked suspiciously like organic bodies-perhaps bacteria - sparsely criss-crossed the sample.

Further delving into this micro-landscape, the same elongated, lath-like crystals seen scattered about the flowstone surface were evident, but in much greater density. At the highest magnification available with this electron microscope, the lath-shaped crystals gave way to sheets made of extremely tiny barrel-shaped crystals. These shapes are typical of larger, up to truck-sized crystals of "apatite."

The biggest surprise was hidden away within a few very narrow folds in the surface. There resided spectacularly spiky rosettes of hydroxylapatite. The normal barrel-shaped crystals were elongated perhaps ten times their expected length. Nearly transparent spikes of the mineral resemble agave plants of the U.S. Southwest.

To my imperfect knowledge, no other cave apatites have been reported to show such extreme forms. Indeed, most apatite crystals described since the mineral was identified in 1856 are much more barrel-shaped and nowhere so spiky.

Although the shapes are unexpected, the origin of the mineral is fairly well known. In caves, it is common for the urine of bat colonies to alter the surrounding calcite cave walls, floors, and decorations to one of several apatite family minerals. The limestone or decorations provide the calcium carbonate and the bat urine provides the phosphate. Such deposits are uncommon throughout the world’s caves and, indeed, in only four other California caves have they been thus far reported.

To the inquisitive mind and keen observer, even the most mundane of caves - such as Rippled Cave - hold many secrets to be reluctantly revealed by Mother Nature.

Note: Comprehension of the size of these tiny structures sometimes escapes us all. One micron is 1/1,000,000 meter or about 1/40,000 inch long. To compare the size of these cave mineral structures to something a bit more understandable, if one could shrink a tennis ball to a micron in diameter, then a "real" tennis ball would be about 38 miles in diameter or roughly equivalent to the straight-line distance from San Jose to San Francisco! In other words, a micron is to the diameter of a tennis ball what the diameter of a tennis ball is to 38 miles.