Cave Minerals, Colors and Other Deposits
Despite what you might think, everything in a cave (even one in Missouri) is not calcite.
Non-speleogenic minerals found here include dogtooth spar (pointed calcite crystals), quartz crystals,
(especially the sort called Potosi druse), gypsum, chert, lead, barite, copper, zinc, iron ores and stains, manganese dioxide, (dark mineral shown above)
even igneous rocks like rhyolite porphyry sometimes form part of cave walls. Fossils can be found in the limestone and
Fortunately for cavers, all these cave minerals have been designated "leaverite"--as in "leave 'er right there."
These minerals are as protected by the Missouri cave law as are more traditional stals. Also included
are archeological and human remains.
Pure calcite is clear, in a variety known as Iceland spar, but
the most common variety is white, the color which results when it is mixed with microscopic
water or air molecules.
Other colors of speleothems are:
- Yellow-tan-brown: impurities of tannic acid from decaying leaves, and other
organic matter, or just plain dirt. This coloring is washed in from the surface.
- Orange-red : probably derived from the cave clay. Cave clay is the residue from
dolomite decomposition, and consists of magnesium and (surprisingly little) iron hydrolyzed
- Red-brown to chocolate brown: Rusty looking, from iron oxides.
- Grey and black: usually manganese, occasionally stains from bat guano, lead or
- Green: impurities of glauconite, a clay/shale, primarily potassium iron silicate, found
between layers of dolomite, and used in textile, sugar and brewing industries, as fertilizer, and a coloring
agent. Around light sources, it may be algae.
- Pink: Probably not calcite at all, but speleothems formed by dolomite. Not common
- Other colors: uncommon, mostly organic or human markings in origin.
Other Cave Deposits:
Stromatolites: Colonial algae preserved as chert
Chert. Chert is a sedimentary rock, largely quartz, (SiO2) but with varying
amounts of calcium silicate and other impurities.It is harder than dolomite. Chert reveals itself as
dark pendants, nodules, and reef structures, mostly noticeable in the cave ceiling. Wherever an
irregular, dark brownish protrusion from the dolomite occurs, it is a good bet it is chert. Missouri
chert has a higher percentage of calcium in it than other microscopically crystalline varieties of
quartz elsewhere; this accounts for the dull finish and easier weathering of this rock than say,
chalcedony, or flint. Chert was deposited in the prehistoric oceans from dissolved molecules of
silica gel clumping together and precipitating; it formed its microcrystalline structure as it
chemically lost its water molecules. Chert is usually full of fossils; in the case of the stromatolites,
it preserves bluegreen algae reefs. Many of the rocks in the cave streams are chert gravel.
Blue-green Glauconitic shale (glauconite clay) With Typical Phreatic Red Clay
Cave Clay. There are two kinds of cave clay. The red, sticky slick clay
(known as unctuous clay) comes from decomposed dolomite, which is why it persists along
ceiling shelves and out of the way places. This clay is red because of the minerals it contains
(mostly magnesium and iron aluminosilicates bound with hydroxyl ions) but chemically it has
much less iron than one might expect from the color. Scientists have found out that temperature,
humidity and pH during decomposition also determines how the clay forms. Bretz (1956)
theorized that cave systems evolved as dolomite soaked in acidic groundwater broke down the
rock, leaving passages filled with clay. If sufficient water continued to flow, eventually the clay
washed out, leaving cave. If the water flow decreased, so did clay transport, refilling cave
passage. In some places this cave clay in Missouri is tens to hundreds of feet deep. We know it
underlays some flowstone caps. Cracked speleothems, which are later recemented, often occur
as a result of the clay cave floor drying out and shrinking, thus lowering the floor level.
The brown mud (actually soil and humus) washes in from the outside, both through the
ceiling and during periods of high water in vadose caves.
Both cave clay and mud preserve animal tracks and bones, both ancient and modern,
Animal tracks have been found in some cave sediments in Missouri, as have both animal and
Photos courtesy Missouri Dept. of Natural Resources
2003 Jo Schaper.
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