Boggs S. Principles of Sedimentology and Stratigraphy
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marginal shallow
fi
DEEP-WATER
DEEP-BASIN
MODEL
prograding supratidal
siment \
subaial evapotes
Figure 7.6
2 Evaporites
205
ALLOW-WATER
DEEP-BASIN
MODEL
shallow-water and
subaerial evaporites
Schematic diagram illustrating th ree models for depo
sition of marine evaporites in basins where water cir
culation is restricted by the presence of a sill.
[Modified from Kendall, A. C., 1979, Subaqueous
evaporites, in R. G. Walker (ed.), Facies models: Geo
science Canada Reprint Ser. 1, Fig. 17, p. 1 70,
reprinted by permission of Geological Association of
Canada.]
from the open ocean takes place only by seepage through the sill or by periodic
overow of the sill. To tal desiccation of the floors of such basins could presumably
occur periodically, allowing the evaporative process to go to completion and
thereby deposit a complete evaporite sequence, including magnesium and potas
sium salts. Thick evaporite deposits could accumulate under these conditions be
cause of continued subsidence.
Applyi.ng the models to ancient evaporite deposits is a challenging task,
and geologists have not always agreed upon the environmental interpretation of
ancient evaporit deposits. Through time, the concept of evaporite deposition has
sww1g from deep-water to shallow-waer deposion; then it swung away from
the tidal sabkha regime to very moderate water depths (Sonnenfeld and Kendall,
1989). See further discussion of evaporite envi ronments in Chapter 11 and, for ad
ditional insight, Busson and Schreiber (1997), Kendall and Harwood (1996),
Melvin (11), Schreiber, Tucker, and Till (1986), Warren and Kendall (1985), and
Warren (1989, 1999).
Physical Processes in De
p
osiHon of Evaporites
Although we tend to regard evaporite deposits as simply the products of chemical
precipitation owing to evaporation, many evaporite deposits are not just passive
chemical precipitates. The evaporite merals have, in fact, been transported and re
worked th same way as the constituents of siliciclastic and carbonate deposits.
Tr ansrt can cur by normal fluid-flow processes or by mass-trsport processes
such as slumps and turbidity currents. Tu rbidity current transport mechanisms
may have been par,ticularly important in deposition of ancient deep-water
206
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
evaporite deposits (Schreiber, Tu cker, and Till, 1986). Therefore, evaporite de
posits may display clastic textures, including both normal and reverse size
grading and various types of sedimentary structures such as cross-bedding and
ripple marks.
Diagenesis of Eva
p
orites
As menoned, most buried evaporite deposits older than about twenty-five mil
lion years have undergone diagenetic alteration. Gypsum is converted to anhy
drite with burial, particularly above 60°C, with a volume loss (from water loss) of
about 38 percent. If exhumation subsequeny occurs, aydrite will hydrate to
gypsum, with accompanying increase in volume. These volume changes are in
part responsible for the formation of nodules and enterolithic structure. Also,
evaporite deposits respond to burial and tectonic pressures by plastic deforma
tion, which destroys original sedimentary structures. Further, deformation can re
sult in folding and diapirism that create large-scale salt diapers (penetration
structures) or salt domes that can rise through sediment for more than 5 km (e.g.,
Jackson, Roberts, and Snelson, 1996). During burial, evaporites may in addition
undergo dissolution, cementation, replacement, and calcitization of sulfates
(Schreiber, 1988; Warren, 1989).
7. 3 SILICEOUS SEDIMENTA RY ROCKS (CHERTS)
Introduction
Siliceous sedimentary rocks are fine-grained, dense, very hard rocks composed
predominantly of the Si0
2
minerals quartz, chalcedony, and opal (in young
rocks), with minor impurities such as siliciclastic grains and diagenetic minerals.
Chert is the general term used for siliceous rocks as a group. Cherts are common
rocks in geologic successions ranging in age from Precambrian to Tertiary; howev
er, they make up only a mor fraction of all sedimentary rocks. They are particu
larly abundant in Jurassic to Neogene (Tertiary) rocks, moderately abundant in
Devonian and Carboniferous rocks, and least abundant in Silurian and Cambrian
deposits (Hein and Parrish, 1987). Geologists are particularly interested in cherts
because of the information they provide about such aspects of Earth history as pa
leogeography, paleooceanographic circulation patterns, and plate tectoni.
Cherts may also have minor economic significance. Silicon is used in the semicon
ductor and computer industries and for making glass and related products su
as fire bricks, although much of this silica may come from quartz sand. Further
more, siliceous deposits occur association with importt economic deposits of
other minerals, such as Precambrian iron ores; uranium, manganese, and phos
phorite deposits; and petroleum accumulations.
Chert is composed maiy of microcrystalline quartz, wi minor chalcedony
and perhaps opal, depending upon age. Cherts can be divided into ree main tex
tural types (Folk, 1974): granular microquartz, consisting of nearly equidimen
sional grains of quartz; average grain size is about 8-10 microns but grain size
may range from <1 to 50 microns (Knauth, 1994); chalcedony (fibrous silica),
formg sheaflike bundles of radiating, extremely thin crystals about 0.1 long;
and megaquartz, composed of equant to elongated grains commonly greater than
20 microns in size.
Figure 7.7 illustrates the texture of microquartz and megaquartz. The silica
that makes up the tests of siliceous organisms is amorphous silica or opal, com
monly called opal-A. Because the remains of siliceous organisms contribute to the
formation of chert, opal A is present in some cherts, particularly those of Te rtiary
3 Siliceous Sedimentary Rocks (Cherts)
207
Figure 7
Fine-textured, nearly equigranular micro
quartz (chert) cut by a vein of much coarser
megaquartz. Source of specimen unknown.
Crossed nicols.
and younger age. Opal-A is metastable and crystallizes in time to opal-CT (dis
cussed below) and finally to quartz (chert). Little opal is present in rocks older
than about 60 Ma (Knauth, 1994). All gradations may be present in siliceous de
posits, from nearly pure opal to nearly pure quartz chert, depending upon the age
of the deposits and the conditions of burial.
Chert is composed dominantly of Si02 but may also include minor amots
of Al, Fe, Mn, Ca, Na, K, Mg, Ti, and a few other elements such as the rare earth
elements cerium (Ce), europium (Eu), and lanthanum (La). Many of these addi
tional elements are contained in impurities such as authigenic hematite and
pyrite,
detrital siliciclastic minerals, and pyroclastic particles. A few elements, in
cluding Fe, Mn, Ni, and Cu, may have precipitated from sea water or pore water
as chert was formed. The amount of Si02 varies markedly in different types of
cherts, rangg from more than 99 percent in very pure cherts such as the
Arkansas Novaculite to Jess than 65 percent in some nodular cherts (Cressman,
1962). Aluminum is commonly the second most abundant element in cherts, fol
lowed by Fe and Mg or by K, Ca, and Na. See Jones and Murchey (1986) for addi
tional details.
Varieties of Chert
Several informal names are applied to chert depending upon color, inclusions,
and texture. Flint is a term used both as a synonym for chert and for a variety of
cher, particularly chert nodules, that occurs in Cretaceous chalks. Jasper is a vari
ety of chert colored red by impurities of disseminated hematite, Jasper interbed
ded wtn hematite in Precambrian iron formations is called jaspilile. Novaculite is
a very dense, fine-grained, even-textured chert that occurs mainly in mid-Paleozoic
rocks of the Arkansas, Oklahoma, and Texas region of the south central United
States. Pon�elanHe is a term used for fine-grained siliceous rocks with a texture
and a fracture esembling those of unglazed porcelain. Siliceous siter is porous,
low-density, light-colored siliceous rock deposited by waters of hot springs an
geysers. Although most siliceou rocks consist predominantly of chert, some con
ain abundant detrital clays or micrite. These impure cherts grade into siliceous
shales or siliceous limestones.
Cherts can be divided on the basis of gross morphology into two principal
s: bedded cherts and nodular cherts. Bedded cherts are further distinguished
by their content of siliceous organisms of various kinds. The principal distin
guishing chracteristics of bedded and nodular cherts are described below.
208
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
Bedded Chert
Bedded chert, also referred to as ribbon dert, consists of laye1·s of nearly pure chert
ranging to several centeters thick that are commonly interbedded wi llime
ter-thick partings or laae of silicus shale (Fig. 7.8). Bedding may be even and
uniform or may show pinching and swelling. Most chert beds lack internal sedi
mentaly structures; however, graded bedding, cross-bedding, ripple marks, and
sole markings have en reported in some cherts. hese kinds of stwctures indicate
that some mechanical transport was involved in the deposition of the sediment that
composes these rks. Bedded cherts are commonly assiated with submarine vol
canic rks, pelagic limestones, and siliciclastic ot carbonate turbidites.
Many bedded cherts are composed dominantly of the remains of siliceous
organisms, which are commonly altered to some degr by solution and recrystal
lization. Bedded chets can be subdivided ott the basis of type and abundance of
siliceous organic constituents into four principal kinds: (1) diatomaceous deposits,
(2) radiolarian deposits, (3) siliceous spicule deposits, and (4) bedded cherts con
taining few or no siliceous skeletal remains.
Diatomaceous Deposits. Diatomaceous deposits include both diatomites and di
atomaceous cherts. Diatomites are light-colored, soft, friable siliceous rocks com
posed chiefly of the opaline (opal-A) frustules of diatoms, a unicellular aquatic
algae (Fig. 7.9). Thus, they are fossil dia tomaceous oozes. Diatomites of both ma
rine and lacustrine (lake) origin are recognized. Marine diatomites are commonly
associated with sandstones, volcanic tuffs, mudstones or clay shales, clayey lime
stones (marls), and, less commonly, gypsum. Lacustrine diatomtes are almost in
variably
associated with volcanic rocks. Diatomaceous chert consists of beds and
lenses of diatomite that have well-developed silica cement or groundmass that
has converted the diatomite to dense, hard chert. Beds of marine diatomaceous
chert comprising strata several hundred meters thick have been ported from
sedimentary sequences such as the Miocene Monterey Formation of California
Figure 7. 8
Thin, well-bedded chert in the Mino Belt Group (Triassic) near Inuyama, Honshu, Japan.
3
Siliceous Sedimentary Rocks (Cherts)
209
Figure 7.9
A loosely packed assemblage of diatoms, siliceous
sponge spicules (rod shaped), and a few radiolari
ans. The diatoms are mainly Coscinodiscus sp. and
Achnoidiscus sp. Holocene sediments from the
northeast Pacific ocean floor; water depth
�4000 m. Photograph courtesy of William N. Orr.
(Garrison et al., 1981) and occur in rocks as old as the Cretaceous. (Diatoms
evolved in the Cretaceous.) Nonmarine diatomaceous deposits have been report
ed in rocks as old as the Eocene (Barron, 1987). When diatomaceous deposits are
converted to quartz chert during diagenesis, the diatom tests are generally de
stroyed by dissolution and recrystallization.
Radiolarian Deposi. Radiolarian deposits consist predominantly of the remains
of radiolarians (e.g., Fig. 7.10), which are ma rine planktonic protozoans with a lat
ticelike skeletal framework of opal. Radiolarian deposits can be divided into radio
larite and adiolarian chert. Radiolarite is the comparatively hard, fine-grained,
chertlike equivalent of radiolarian ooze, that is, indurated radiolarian ooze.
Figure 7. 10
Radiolarian chert from the Otter Point Formation Uurassic), south
western Oregon. Most of the small, rounded bodies in this sample
are radiolarians. The large fracture at the lower right is filled with
silica (quartz) cement. Ordinary light photograph. (Photograph
courtesy of Shelia A. Monroe.]
210
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimenta Rocks
Figure 7.1 1
Radiolarian chert is well-bedded, microcrystalline radiolarite that has a well
developed siliceous cement or ground mass. Radiolarian cherts are ommonly as
sociated with tuffs, mafic volcanic rocks such as pillow basalts, pelagic Hmesto�es,
and turbidite sandstones that ate a deep-water oigirt. These bedded cherts,
particularly those that exhibit a "pinch and swell" texture, are commonly called
ribbon cherts. On the other hand, me radiolarian cherts are assiated with mi
critic limestones and other rocks that suggest deposition in water perhaps as shal
low as 200 m (Iijima, tnagaki, and Kakuwa, 1979). Radiolarians tend to survive
silica diagenesis more effectively than do d�atoms; therefore, they are ommon
components of many quartz cherts (Hein, Ye h, and Barron, 10).
Siliceous Spicule Deposits. Spicularite (spicue) is a silkeous r composed prin
cipally of the siliceous spicules of invertebrate organisms, particularly sponges (see
Fig. 7.9). Spicularite is loosely cemented in contrast to spicular chert, which is hard
and dense. Spicular cherts are mainly marine in crigin d are associated with
glauconitic sandstones, black shales, dolomite, argillaceous (clayey) limestos,
and phosphorites. They are not generally associated with volcanic rks and are
probably deposited mainly in relatively shallow water a few hundred meters deep.
Nonfossiliferous Cherts. Many bedded chert deposits have been described that
contain few or no recognizable remains of siliceous organisms. Some of these
ported occurrences of fossil-barren cherts may simply be the result of inadequate
microscopic examination of the cherts which, upon closer examination, might be
found to contain siliceous organisms. Others have been examined closely and
clearly contain few siliceous organisms. Cherts this latter group include most
cherts associated with Precambrian iron-formations, as well as some Phanerozoic
cherts. The origin of these cherts is still poorly understood (see discussion under
"Origin of Chert").
Nodular Chert
Nodular cherts are subspheroidal masses, lenses, or irregular layes or bodies that
range in size from a few centimeters to several tens of centeters (fig. 7.11 . They
commonly lack internal structures, but some nodular cherts wnin silicified fos
sils or relict structures such as dding. Colors of these (herts vary from green to
tan and black. Nodular cherts typically occur in shelf-type carnate rks where
they tend to be concentraed afong certain horizons parallel to bedding. They
occur also in some sandstones, shales, deep-sea days, lacustrine sediments, and
evaporites. Nodular cherts originate by
diagenetic replacement (e "Diagenesis
of Chert"). Diagenetic origin is clearly demonstrated in many nodules by the pres
ence of partly or wholly silicified remas of calcareous fossils or ids.
Nodular chert in limestones of the Helena Formation (Precam
brian), Glacier National Park, Montana.
Origin of Che
3 Siliceous Sedimentary Rocks (Ches) 211
Two principal questions must be answered with respect to e origin of chert:
What were the sources of the silica, and what mechanisms have operated in the
past to extract silica om water, principally seawater, to form chert? e answers
to these questions are own to a reasonable degree (see reviews by Carozzi, 1993;
Hesse, 1990; Knauth, 1992, 1994); however, some aspects of the origin of chert,
such as the mechanism responsible for deposition of Precambrian chert, remain
unresolved.
Sources of Silica
Most bedded chert is present marine sedimentary rocks. To understand e ori
gin of chert requires that we have some knowledge of sources of silica and that we
understand the mechanisms by which silica is extracted fm seawater to form
ert. In average river water, the concentration of silica in transport (from conti
ntal weathering sites) as H4Si04 is about 13 ppm. addition to silica transport
to the oceans by rivers, silica is added to the oceans through reaction of seawater
with hot volcanic rocks along mid-ocean ridges and by low-temperature alteration
of oceanic basalts and detrital silicate particles on the seafloor (called halmyroly
sis), as described in Chapter 1. Some silica may also escape from silica-enriched
pore waters of pelagic sediments on the seafloor. These silica sources are summa
rized in Figure 7.12, which also depicts the pathway of silica diagenesis from opal
A to quartz chert (to be discussed). Despite contributions of silica from these
various sources, the silica concentrations in different parts of the ocean range from
less than 0.01 ppm to a maximum of about 11 ppm; the average dissolved silica
content of the ocean is only 1 ppm (Heath, 1974). Cleay, silica is constantly beg
moved by some process and has a relatively short residence time in the ocean.
Silica Solubili
Solubility
studies show that the solubility of silica in seawater differs for different
silicate minerals. The solubility of Si02 at 25"C and normal ocean pH ( �7.8-8.3)
ranges from �6 to 10 ppm for quartz to �60 to 130 ppm for amorphous or non
crystalline varieties of silica such as opal (Krauskopf, 1959; Morey, Fournier, and
RIVER INPUT FROM
CONTINENTS
Fire 7. 12
Sources of dissolved silica in sea
water. [After Riech, V., and U. von
Rad, 1979, Silica diagenesis in the
Atlantic Ocean: Diagenetic poten
tial and transformations, in Ta l
wani, M., W. Hay, and W. B.
Ryan (eds.), Deep drilling results in
the Atlantic Ocean: Continental
margins and paleoenvironment:
Amer. Geophysical Union, Maurice
Ewing Series, v. 3, Fig. 2, p. 322,
modified from Heath, G. R., 1974,
Dissolved silica and deep-sea sedi
ments, in W. W. Hay (ed.), Studies
in paleooceanography: Soc. Econ.
Paleontologists and Mineralogists
Spec. Pub. 20, Fig. 7, p. 81 ,
reprinted by permission of SEPM,
Tulsa, Okla.
212
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimenta Rocks
Rowe, 1962, 1964; Iller, 1979). Knauth (1994) suggests a lower limit of 4 ppm for
quartz solubility. erefore the ocean, which has an average dissolved silica con
tent of only l ppm, is grossly undersaturated with respect to silica, in sharp con
trast to the saturated state of the surface ocean with spect to calcium carbonate.
This fact raises a very intriguing question. What mechanism (or mechanisms) is
(are) capable of removing silica from highly undersaturated ocean water to form
chert beds and maintain the low concentrations of dissolved silica in the ocean?
The solubility of silica is affected by both pH and temperature. Change in
solubility of silica with pH is illustrated in Figure 7.13A. Solubility changes only
slightly with increase pH up to about 9, but it rises sharply at pH values above 9.
Solubility increases with increasing temperature in essentially a linear fashion,
and solubility at 100"C is nearly 3 to 4 times that at 25°C (Fig. 7.138). Solubility also
increases with increasing pressure. Clearly, the greater the solubility of silica
under a given set of conditions, the less likely it will be to precipitate to form chert.
See Dove and Rimstidt (1994) for an extended, and more rigorous, discussion of
silica solubility.
Silica Extraction from Seawater
Chemical Extraction. In a laboratory experiment at 200C lasting two years,
Mackenzie and Gees (1 971) precipitated quartz from seawater containing 4.4 ppm
dissolved silica onto nucleation surfaces of cleaned quartz grains. Nonetheless,
silica in soluon under natural conditions of temperature and pH in the ocean ap
parently does not readily crystallize to form quartz, even from solutions that have
Figure 13
The solubility of silica as a function of (A) pH and (B) temperature. The solid line in A
shows the variation in solubility of amorphous silica as determined experimentally. The
upper dashed curve shows the calculated solubility of amorphous silica, based on an as
sumed constant solubility of 120 ppm Si02 at a pH below 8. The lower dashed line is the
calculated solubility of quartz based on the approximate known solubility of 6 ppm Si02
in neutral and acid solutions. [A. after Krauskopf, K. B., 1979, Introduction to geochem
istry, 2nd ed., Fig. 6.3, p. 133, McGraw-Hill, New York. B. after Fournier, R. 0., 1970, Sili
ca in thermal waters: Laboratory and field investigations: Proc. International Symposium
on Hydrochemistry and Biochemistry, Tokyo, p. 122-1 39.]
3 Siliceous Sedimentary Rocks (Cherts)
213
silica concentrations exceeding the solubty of quartz. Therefore, it is unlikely
that chert, which consists of microcrystalline quartz, can be precipitated by inor-
ganic processes from highly undersaturated seawater. Chert might be precipitated
in some local basins where waters are saturated with silica, owing perhaps to dis-
solution of volcanic ash or other processes related to volcanism (e.g., Hesse, 1989;
Ledesma-Vazquez et al., 1997). Also, some silica may be removed from seawater
in e open ocean by precipitation or adsorption onto clay minerals or other sili-
cate parcles, as in the Mackenzie and Gees (1971) experiment; however, such
processes likely caot account for the many bedded successions of nearly pure
chert present in the geologic record.
Biogenic Extction. Removal of silica from ocean water by silica-secreting oan
isms to build opaline skeletal structures appears to be the only mechanism capa
ble of large-scale silica extraction from undersaturated seawater. This biologic
process has operated since at least early Paleozoic time to regulate the balance of
silica
in the ocean. Radiolarians (Cambrian/Ordovician-Holocene), diatoms (Cre
ta ceous-Holocene), and silicoflagellates (Cretaceous-Holocene) are microplank
ton that build skeletons of opaline silica (opal-A). These siliceous microplankton
(particularly diatoms and radiolarians) have apparently been abundant enough in
e ocean dung Phanerozoic me to extract most of the silica delivered to the
oceans by rock weathering and other processes. Diatoms are probably responsible
for the bulk of silica extraction from ocean waters in the modern ocean and during
much of the past fifty million years (Calvert, 1983; Knauth, 1994); however, radio
laans were the important users of silica in the Phanerozoic oceans of Jurassic and
older
ages. Heath (1974) calculates that the residence time for dissolved silica in
the ocean ranges from o hundred to three hundred years for biologic utilization
to 11,0 to 16,000 years for incorporation into the geologic record-a very short
time from a geologic point of view.
Box 1 Formation of Biogenic Che
The mechanism by which the opaline tests of siliceous organisms are convert
into chert is illustrated by pathway A in Figure 7.1.1. While silica-secreting
organisms are alive, their siliceous (opal-A) skeletons undergo little dissolu
tion in highly undersaturated and corrosive seawater. Cell walls are protected
by some physicochemical system, such as armoring by metal ions, related to
their vital activity (Lewin, 1961
)
. After death, this protective system is disrupt
ed and dissolution begins. In areas of the ocean where silicious organisms
ourish, the rate of production of siliceous skeletons may be so high that
they cannot all be dissolved as rapidly as they are produced. Under such
cond itions, a sufficient number of the siliceous skeletons may survive total
dissolution
to accumulate on the seafloor as siliceous oozes (sediments con
taining at least 30 percent siliceous skeletal material and commonly more
than percent). After burial by additional siliceous ooze or clayey sedi
ment, these opaline skeletal materials continue to undergo solution; howev
er, after burial much of the dissolving silica is trapped in the pore spaces of
the sediment and does not escape back to the open ocean. The pore waters
thus become increasingly enriched in silica, leading eventually to precipita
tion of chert.
Dug e process of transformaon of biogenic opal to chert, opal-A may
not convert directly to quartz chert, which is microcrystalline quartz, but com
monly goes through intermediate, metastable phase: opal-CT (Fig. 7.1.1). Al
thou called opal-CT, is silica phase is composed maly of low-temperature
214 Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
0
o_
�
i
40
E
�
:
·
80
-
�
Figure 7.1.1
:
0
·:
c
·
�
Increasing Water Depth
Siliceous
Oozes
Epicontinental
Cherts
Zone of
compaction
Restricted
fluid movement
owing to
compaction
Recstallization
metamorphism
Schematic diagram showing major silica phases and their possible diagenetic transfor
mations. Ve rtical dimension represents qualitative burial depth with associated increase
in temperature and loss of porosity and permeability. Horizontal dimension represents
qualitative water depth of the initial environment. In general, deep-sea siliceous oozes
lie to the left of the diagram, whereas epicontinental deposits lie toward the right. Dia
genetic path A represents silica initially deposited as opal-A (diatoms, radiolarians),
which subsequently transforms to opai-CT and then microquartz by means of solution
reprecipitation steps. Path C represents early diagenetic cherts in which microquartz
forms during shallow burial. Path B represents a possible pathway for chert formation
under conditions intermediate between A and C. Megaquartz forms by metamorphic
recstallization of microquartz or by direct growth into voids (suggested by the
"spike") at any stage of burial. Fibrous silica can grow in vugs and fractures at all burial
depths. (After Knauth, L. P., 1994, Petrogenesis of chert, in Heaney, P. J., C. T. Prewitt,
and G. V. Gibbs (eds.), Silica: Physical behavior, geochemistry and materials applica
tions: Mineralogical Society of America Reviews in Mineralogy, v. 29, Fig. 4, p. 239, re
produced by permission.)
cristobalite disordered by interlayered tridymite lattices. Cristobalite and
tridymite are metastable varieties of quartz that alter with time to quartz. Opal
CT may occur in open spaces in sediments as lepispheres, which are microcrys
talline aggregates of blade-shaped crystals. It c also form as nonspherulitic
blades, rim cements, and overgrowths, and as a massive cement (Maliva and Siev
er, 1988a). Note at the transformation of opal-A to opal-C is a solution-reprecipi
tation process; that is, opal-A dissolves to generate silica-rich pore waters from
which opal-CT precipitates. m, opal-CT is converted into microquartz, also
by solution-reprecipitation. Finally, microquartz is transformed to megaquartz as
burial temperatures approach those of metamorphism. Megaquartz, as well as