Quite a few years
ago, I used to use diatomaceous earth for filtering water in fish aquariums.
It makes an excellent filter medium that is capable of removing the smallest
particles. It can even filter out parasites. Although I knew that it was
produced from the fossil skeletons of an aquatic organism, I never gave
much thought to everything that was necessary to develop, mine, and process
this material, As far as I was concerned, it just came from a store shelf.
After having an opportunity to see the Celite Corporationís diatomite mine
in Quincy, Washington, I became fascinated with all that must have occurred
to create the deposits that they were mining and how they processed it
to achieve the end product. It is the goal of this paper to answer those
questions.
Although diatoms
appear plant like, scientists have determined that these single-celled
organisms are neither plant nor animal. Diatoms are classified with the
single-celled protozoans, molds, and fungi into a separate group called
the Kingdom Protista (Burnett, 1993).
Diatoms live in many diverse environments. They can be found in the oceans,
lakes, streams, salty inland seas, and brackish estuaries. Diatoms can
be found in almost any body of water where there are adequate nutrients.
Most diatoms use photosynthesis to produce their energy, so they also need
sunlight. There are some diatom species that do not contain chlorophyll.
These diatoms must acquire energy by some means other than photosynthesis.
Although diatoms can live in environments with a wide range of temperatures,
they are more prolific in colder waters. They are very abundant in the
polar oceans. Most diatoms live in the open water column at or near the
surface (Werner, 1977). When they die they sink to
the bottom. The soft tissues decay leaving behind a fossil skeleton.
Most diatom
fossils are found in Eocene and Miocene sedimentary rocks. The oldest known
diatoms that have been definitely identified and dated are from the Lower
Cretaceous (Burnett, 1993). Fossil
diatoms which are subjected to pressure may recrystallize into metamorphic
rock and this may explain why older fossils are not found (Compton,
1991). There is be an active debate in the scientific community, as
to when diatoms first appeared. In 1951 G. Dallas Hanna, a pioneer in the
study of diatoms, speculated that the history of diatoms must go back farther
than the Cretaceous. His assumption was based on the fact that by the time
of the Cretaceous, diatoms were already very abundant, highly organized
and of many diverse forms (Burnett, 1993).
Recent work, using inferred phylogenetic trees from 18S rDNA sequences,
indicates that diatoms have their origins somewhere around 238 Ma to 266
Ma.
Diatoms consists
of a membrane supported and protected by two half-cell walls or valves.
The two valves and their connecting band form the diatoms silica skeleton
called a frustule. These two valves, the epitheca and the hypotheca are
different sizes. The epitheca, being the larger of the two, slightly overlaps
the rim if the hypotheca like a lid (Burnett,
1993). The organisms extract silica from the water to build their frustule.
Typically, the frustules exhibit complex lattice-work patterns and partitions
of great variety and complexity. Since the total thickness of the each
valve wall is only a few microns, it results in an integral structure that
is highly porous on a microscopic scale (Hanna, 1951).
The frustule of most diatom species are between 50 and 150 microns in diameter
(Benton, 1983).
The frustule, or silica
skeleton of a diatom is made up of amorphous silica which has the same
chemical composition as opal (Hanna, 1951). The chemical
formula for amorphous silica is SiO2ïnH2O.
The water content usually ranges from four to nine percent, but it can
be as high as twenty percent. Although this form of silica does not form
in a crystal lattice the structure is highly ordered. Individual silica
spheres arrange themselves in hexagonal or cubic closest packing, water
and/or air taking up the space in the voids (Klein and
Hurlbut, 1985).
Diatoms can
reproduce both sexually and asexually. During sexual reproduction diatoms
produce non-siliceous gametes which are released into the surrounding water.
When two gametes join together they form a complete zygote, enlarge, and
build a frustule.During asexual reproduction the two halves of the frustule
separate and each half generates a new hypotheca. When the smaller hypotheca
from the original individual generates a new hypotheca, it becomes the
epitheca, or larger half. This causes the new individuals to get progressively
smaller. Although asexual reproduction does cause the line to become progressively
smaller, it does allow diatoms to reproduce very rapidly in blooms. This
allows them to quickly take advantage of favorable changes in environmental
conditions such as an increase in dissolved silica. Sexual reproduction
then serves not only the purpose of combining the genetic material from
different individuals, it also allows the progeny of asexual reproduction
cycles to produce offspring that will grow to full size (Werner,
1977).
There is a wide
range of estimates on the number of diatom species that have existed. Many
estimates place the number as high as 100,000 to 200,000 different species.
Of this great number only about 25,000 bona fide species have been actually
cataloged and described (Werner, 1977).
Diatomite is
the specific name given to fossil diatom deposits that are large enough
and pure enough that they are of potential commercial value (Benton,
1983). Although diatoms inhabit many different environments throughout
the world they are in most instances, not accumulating in the concentrations
that were necessary to form the Miocene and Pliocene age deposits that
are being mined throughout the world today (Burnett,
1991).
Two primary
factors that affect the formation of diatomite deposits are the rate of
accumulation of diatoms frustules, and the rate at which other sediments
are accumulating in conjunction with them. The amount of available silica
in solution is typically a limiting factor in the reproductive rate of
diatoms and therefore controls their rate of accumulation. This may explain
why most of the known diatomite deposits are Miocene and Pliocene in age.
This was a time of increased volcanism throughout the world (Hanna,
1951). Most sources of silica are not very soluble in water, but the
silica in volcanic ash dissolves comparatively easily (Burnett,
1993). Considering the extent of volcanism during the Miocene and Pliocene
and the large populations of diatoms that must have been living, diatomite
is less abundant than might be expected. Diatom fossils are present in
many sedimentary rocks of this age, but other sediments accumulated in
conjunction with them in such great quantities that the diatom fossils
only comprise a small proportion. Two deposits of particular interest are
the Celite Corporationís mines in Lompoc, Ca. and Quincy, Wa. The Quincy
deposits are of interest because I have been to the mine. The Lompoc deposits
are of interest because of their tremendous size and contribution to global
production.
In the area
of Quincy, Washington two separate Miocene diatomite deposits have been
mined commercially (Benton, 1983). At various times
throughout the Miocene, lava flows of the Wanapum Basalt blocked off streams
channels causing lakes to form (Niemi, 1981). These lakes provided an ideal
environment for the growth of diatoms. The volcanic eruptions that provided
the lava to create the lakes would have also contributed volcanic ash to
the drainages that fed the lakes. The volcanic ash provided an abundant
source of silica, which is needed for the rapid growth of diatom populations.
Iron can also be a limiting factor in the growth of diatoms. The lakes
were formed in basalt which is relatively high in iron and weathers easily.
Many such lakes probably formed throughout Eastern Washington at this time.
Of these lakes, most would have received large enough amounts of terrigenous
sediments. The sedimentary rocks that formed in most of these basins may
contain fossil diatoms, but not in sufficient concentrations to be considered
diatomite (Mackin, 1961). Only the lakes which received
minimal amounts of terrigenous sediments formed diatomite deposits. The
Squaw Creek Diatomite and the Quincy Diatomite both formed in these types
of lakes (Niemi, 1981).
The Squaw Creek
Diatomite occurs at the base of the Rosa Member of the Wanapum Basalt.
The Squaw Creek Diatomite is the older of the the two deposits in the Quincy
area. This deposit was formed during a period of volcanic inactivity after
the formation of the Frenchman Springs Member. Basalt flows of the Rosa
Member capped off the deposit probably while it was still being formed.
This is evidenced in the Rosa Peperite where basalt flowed into the diatomaceous
sediments which were apparently young and not well consolidated. The Quincy
Diatomite occurs at the base of the Priest Rapids Member of the Wanapum
Basalt (Carson, et al, 1987). This deposit
is currently being mined by the Celite Corporation.
The Celite Corporation
also operates the larger of the two diatomite mines in Lompoc, California.
The Lompoc deposits are the worlds largest source of diatomite being mined
today (Burnett, 1991). The diatoms
of the Lompoc area occur in the Sisquoc and Monterey Formations which are
upper Miocene to Lower Pliocene in age. These deposits were formed during
a six or seven million year period that spans the Late Miocene to Early
Pliocene (Compton, 1991). Various parts of the Sisquoc
Formation are discontinuously exposed at the surface for more than 15 miles
in a series of closely-spaced folds. Individual diatomite beds range from
only a few centimeters up to 15 meters and the entire deposit is hundreds
of meters thick (Burnett, 1991).
During the Late
Oligocene and Early Miocene the intersection of the East Pacific Rise with
the North American Plate created a region of extensional tectonic activity
along the coast of Southern to Central California. This tectonic activity
created the sub-marine basin in the Pacific ocean, adjacent to the coast.
The sediments that make up the Sisquoc Formation and the underlying Monterey
Formation were deposited in this basin (Compton, 1991).
Throughout the remainder of the Miocene siliceous frustules of diatoms
were deposited in the basin with very little accumulation of terrigenous
sediments. The large populations of diatoms necessary to form these deposits
were enhanced by two additional factors. First, as mentioned earlier the
Miocene was a period of increased volcanism. Volcanic activity contributed
ash to the water, increasing the dissolved silica. Second, the period from
16 to 12 million years ago represents a major global cooling event (Flower
and Kennett, 1993). These cooler temperatures would have been favorable
for the growth of large populations of diatoms.
During the Early
Pliocene, the Santa Ynez Mountains were uplifted adjacent to the basin.
This resulted in a quadrupling in the rate of overall sedimentation, most
of which was terrigenous on origin. The uplift of the Santa Ynez ended
the formation of the relatively pure diatomite deposits by contaminating
the basin with large amounts of terrigenous sediment being eroded off the
newly formed mountains (Compton, 1991).
This dramatic
increase in sedimentation greatly increased the overburden pressure being
applied to the Monterey Formation. The increased pressure coupled with
a high geothermal gradient, probably brought about by thinning of the crust
and an upwelling of the asthenosphere in this extensional environment,
caused diagenesis of the diatomite in the lower sixty to seventy percent
of the Monterey Formation. This process transformed the diatomite to cristobalite.
The remainder of the Monterey Formation and the Sisquoc formation were
not buried deep enough or heated sufficiently for diagenesis to occur.
From the Early Pliocene to the late Pleistocene the Monterey and Sisquoc
Formations were tectonically folded, tilted and uplifted. Diatomite was
then exposed at the surface by the erosion of overlying sediments (Compton,
1991).
There are currently
12 diatomite producing facilities in the United States which are operated
by six different companies. Although underground mining of diatomite has
occurred in the past (Oakeshott, 1957), all current
U.S. mining is done by open pit methods (Lemons, 1996).
All of the active diatomite mines is the U.S. are freshwater lake deposits
except the marine deposit at Lompoc, California. The mining and processing
of diatomite is unusual in that care must be taken not to destroy the structure
of the individual diatom frustules. Diatomite can not be subjected to excessive
attrition in the process of milling and conveying. Unprocessed diatomite
is referred to in the industry as crude. It is typically mined using bulldozers
and trucks (Oakeshott, 1957).
The crude is
transported from the mine to processing plants which are usually located
nearby. Crude has a density of between 320 and 640 kg. per cubic meter
and a moisture content of thirty to sixty percent. It is first crushed
to a size of about one to two centimeters and furnace dried to reduce the
moisture content to about fifteen percent. Diatomite is usually made up
of a number of different species of diatoms. After initial drying the crude
is cleaned and sorted by particle size. This is done in series of cyclone
classifiers. The classifiers use hot gasses to sort the different particle
sizes and further dry them at the same time.. After being sorted and dried
the crude may be bagged and sold as natural diatomite. Although the diatomite
has been sorted, each grade still has a considerable size distribution.
For this reason most diatomite products are further processed (Benton,
1983).
Diatomite products that are further processed are called calcined and
flux calcined diatomite. Most diatomite products are used in filtering
applications. With a large particle size distribution, the smallest of
the particles occupy space between larger particles. This reduces the rates
of flow through the filter. The purpose of calcining is to reduce the particle
size distribution. The use of the term calcined is ingrained in the diatomite
industry and markets so it is used frequently, but diatomite is actually
sinterized not calcined (Benton, 1983).
Sintering reduces
the size distribution by melting the smallest particles together. To produce
calcined diatomite the natural product is heated to between 900° C.
and 1100° C. The high temperatures burn off organic contaminants, and
shrink and harden the individual particles. Some of the diatom frustules
are sintered into small clusters. The resulting calcined diatomite has
a density of about 125 to 150 kg. per cubic meter. Flux calcined diatomite
is produced using the same methods except a fluxing agent is added before
heating. Soda Ash in concentrations of between three and seven percent
is usually used as the fluxing agent. The diatomite is heated to around
1200° C. slightly higher than for straight calcined products. Varying
the temperature, amount of flux added and the processing time controls
the particle size distribution. Flux calcined diatomite has a wide density
range depending on the processing. It varies from about 150 to 300 kg.
per cubic meter (Benton, 1983).
In 1996 the U.S.G.S. conducted a production survey of all twelve of the U.S. producers. The annual production in the United States for 1995 was 687,000 tones (table 1) (Lemons, 1996). This represents an increase of twelve percent from 1994. Although domestic production did increase in 1995, it has not increased significantly over the last 30 years. An extrapolation from the 1991 Minerals Commodity Report on Diatomite published by the California Department of Conservation projected 1995 production at 768,000 tonnes. This is 81,000 tonnes, or twelve percent higher than the actual 1995 production reports. During the four years since the 1991 report was published world production of diatomite has decreased by 240,000 tonnes (Lemons, 1996). As world production and probably demand have decreased, U.S. production has increased slightly, but it is not increasing at even the modest rates projected only 4 years earlier.
Diatomite has been used in hundreds of applications since it was first commercially mined in the 1800ís. In 1995 eighty-four percent of all diatomite produced in the U.S. was used in three principal areas (table 2); as a filter medium, as a filler, and as an insulator. seventy percent of all production was used in filtering applications (Lemons, 1996). These applications vary widely from filtering beverages and food products to swimming pools. As a filler, diatomite is used many applications primarily because it has a low density and is relatively inert. It is used as an filler/extender in paints because of its unique ability to trap pigments helping to distribute the color evenly throughout the mixture. Diatomite is also used as a filler in pharmaceuticals and many other chemical applications (Burnett, 1991). Low-grade diatomite that contains enough clay to act as a bonding agent has been used for manufacturing bricks. This was one of its first commercial applications (Oakeshott, 1957).
World reserves of diatomite are estimated at 800 million tonnes (Kesler, 1994). At current rates of consumption this is enough to last for almost 600 years. The Celite Corporationís mine at Lompoc, Ca. covers an area of eleven square kilometers to a depth of 200 meters. This mine alone could probably meet world demand for the next 150 years. One of the threats to reserves and the reserve base is growing urbanization. Land covering otherwise commercially mineable deposits in Ventura, Los Angles and Orange counties has been developed for industrial and residential use. It is unlikely that these deposits will be extracted in the foreseeable future (Burnett, 1991).
In any mining operation care must be taken to preserve and restore the
natural environment. Because natural diatomite is composed of mostly amorphous
silica it does not present as serious an environmental or health risk as
crystalline forms of silica. Unlike crystalline forms of silica, diatomite
is not classified as a potentially carcinogenic (Burnett,
1991).
Reclamation is an important part of commercial mining. Public perception
drives public policy. This becomes especially important in areas like Lompoc,
Ca. where mining is occurring in close proximity to a large, growing urban
center. Diatomite is essentially inert and therefore poses little or no
threat to the environment in both active and reclaimed mines. Because of
the high absorption capacity of diatomite, it retains moisture which aids
in the re-vegetation process where mine pits are being reclaimed. This
high absorption capacity may also help in controlling runoff of precipitation.
This would significantly reduce erosion, which can be a difficult problem
when trying to restore topography. In Los Angeles County a diatomite quarry
on the Palos Verdes Peninsula was closed in 1956 after 27 years of operation.
After being closed the quarry pit served as a sanitary landfill for 8 years.
The county then created the South Coast Botanic Garden over the landfill
(Burnett, 1991). Based on my own observations
at the Celite Corporationís mine in Quincy, Washington, it appears that
the Celite Corporation is doing an excellent job returning the land to
its original state.
Safe working and living environments can be maintained in and around
diatomite mines by reducing the amount of material that becomes airborne,
and controlling exposure times. In the processing plants, workers are exposed
to diatomite products that have been sinterized. In the sintering process
some of the amorphous silica recrystallizes to cristobalite. This mineral
is considered to be carcinogenic. When Inhaled in sufficient quantities,
over prolonged periods, it is known to cause fibrotic lung disease. By
maintaining adequate ventilation, keeping work areas clean, and using respirators,
health problems of this type can be eliminated. A twenty-one year long
study reported on by the Mansville Corporation (Celite Corp.) demonstrated
that by taking these precautions, health problems related to fibrotic lung
disease can be practically eliminated (Benton, 1983).
Burnett, J. L., 1991, Mineral Commodity
Report -Diatomite-: California Department of
Conservation, Division of Mines and Geology, Special Publication 111,
26 p.
Burnett, J. L., 1993, Diatoms-The Forage of The Sea: California Geology, v. 44 no. 4, p. 75-81.
Benton, W. E., 1983, Economics of Diatomite: AIME pre-print no. 83-363, 15 p.
Carson, R. J. et al, 1987, Geology of the
Vantage area, south-central Washington: An
Introduction to the Miocene flood basalts, Yakima Fold Belt and the
Channeled Scabland: Geological Society of America Centennial Field Guide
? Cordilleran
Section, p. 357-362.
Compton, J. S., 1991, Porisity reduction and burial history of siliceous rocks from the Monterey and Sisquoc Formations, Point Pedernales Area, California: Geological Society of America Bulletin, v 103, p. 625-636.
Flower, B. P., Kennett, J. P., 1993, Relations
Between Monterey Formation Depositition and Middle miocene Global Cooling:
Naples beech Section, California, Geology,
v 21, p. 877-880.
Hanna, G. D., 1951, Diatom Deposits: California
Division of Mines Bulletin 154, p. 281-290.
Kesler, S. E., 1994, Mineral Resources, Economics and the Environment:
Macmillan
College Publishing Company, New York, N.Y., 391 pp.
Klein, C., Hurlbut, C. S., 1985, Manual of Minerology: 20th ed., John Wiley & Sons, New York, N.Y., 596 pp.
Kesler, S. E., 1994, Mineral Resources, Economics and the Environment, Macmillian College Publishing Company, New York, N.Y., 391 pp.
Lemons, J. F., 1996, Diatomite: U.S.G.S. Minerals Yearbook 1995, http://minerals.er.usgs.gov /minerals /pubs /commodity /datomite /250495.pdf, 4 p.
Mackin, J. H., 1961, A Stratigraphic Section in
the Yakima Basalt and Ellensburg
Formations in south-central Washington: Washington Division of Mines
and
Geology Report of Investigations 19, 45 p.
Niemi, W. L., 1981, The Identification and Stratigraphic
Correlation of Basalt Aquifers in the Southern Half of The Quincy Basin,
Grant County, Washington, Using
Borehole Geophysics: University of Idaho, M. S. Thesis.
Oakeshott, G. B., 1957, Diatomite: Mineral Commodities
of California, Department of
Natural Resources Division of Mines Bulletin 176, p. 183-193.
Werner, D. ed., 1977, The Biology of Diatoms, Botanical
Monographs, University of
California Press, v. 13, 498 pp.