Both casts and replacements @shotgunner. Right ? To get corallite patterns a replacement is required.
Odd, the center usually looks totally replaced w/no patterns, as you move to the edge natural patterns start to show.
Some coral has patterns throughout.
However, about all coral has this fingerprint as seen in the glare of the black coral dead center.
About any piece of Fl/Ga coral.
Said 'fingerprint' can be seen on arrowheads. That is how people ID coral artifacts. It has a thumbprint. Rare characteristic of any rock.
"They are found in ancient ocean beds, where silica rich groundwater has percolated through them
and over time has replaced it's calcium carbonate skeleton with a hard variety of Chalcedony."
"Agatized/fossilized coral is found in several Florida locations and may have been formed when
runoffs of silt rich in clay and silica buried an Eocene aged Coral reef"
"Much of Florida’s agate, including the Tampa Bay agatized coral, formed in the Oligocene-Miocene Hawthorn Group sediments" ha, Georgia clay washed down and coated the flatlander's coral reefs.
" Agatized coral occurs when silica in the ocean water hardens, replacing the limy corals with a form of quartz known as chalcedony.
This long process (20-30 million years) results in the formation of a "pseudomorph,"
meaning that one mineral has replaced another without having lost its original form."
Note center chip piece. Tubes(always parallel) are broken off and laying in several directions and then coated.
Many specimens have broken tubes and then coated at the bottom of pseudomorphs(hollow corals).
A good example of broken tubes fallen to the bottom and coated(with a hard variety of chalcedony).
I believe chalcedony is the answer to dissolved silica silicification-
Sabre52, you may find this interesting.
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Geochemistry[edit]
Structure[edit]
Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz.[11] More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite.[2] The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.[12] The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.[13][14]
Solubility[edit]
Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio.[citation needed] It has also been suggested that the higher solubility is due to the moganite component.[12]
Solubility of quartz and chalcedony in pure water[edit]
This table gives equilibrium concentrations of total dissolved silicon as calculated by PHREEQC (PH REdox EQuilibrium (in C language, USGS)) using the llnl.dat database[citation needed].
Temperature Quartz solubility (mg/L) Chalcedony solubility (mg/L)
0.01 °C 0.68 1.34
25.0 °C 2.64 4.92
50.0 °C 6.95 12.35
75.0 °C 14.21 24.23
100.0 °C 24.59 40.44
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We have lots of water and 30C(86F) temperatures.
Lots of very finely grained silica from mega tons of micro fine diatom structures.
I saw no mention of acidity and alkalinity. Certain it has impact.
Temperature and grain size may be the most important factors.
Finally had time to learn more about silicification in the SE US.
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Next subject is Moganite. Ha, common in Florida.:
A recent article by Peter Heaney and Jeffrey Post in Science added insult to injury. They reported finding moganite in nearly all of the 150 samples of fine-grained quartz they tested from the Smithsonian's mineral collection. Some samples contained over 75% moganite mixed with the quartz. Chert had the most moganite. Flint contained 13 -17% moganite. Agate had 5 to 20% moganite. ----------------Silicified corals from the Tampa Bay area in Florida averaged 20% morganite.-------------- The only samples found with no moganite were jaspers from iron formations, the weathered outer rinds of agates and Arkansas novaculite. Clearly moganite is a very overlooked mineral.
Moganite discovered in 1984 !!
Above quote from this Moganite article:
Moganite - A Common Mineral with a Disapproved Name
Mineralogists assume that they have described the common minerals of the crust. They also pride themselves on having a rigorous international system for approving new mineral names to assure the names that get into print really do represent uniquely different minerals. Recently both of these assumptions have been thrown into disarray by a mineral called' "moganite". Moganite turns out to be so common that virtually every rockhound has unknowingly found, polished or bought it.
Moganite refers to a mineral closely related to quartz. Quartz is Si02 and crystallizes in the hexagonal system. Moganite is also Si02, but it crystallizes in the monoclinic system. It was discovered in 1976 by a group of geologists lead by the Russian O.W. Florke. This team was studying the rocks near Mogan in the Canary Islands and found it as fine-grained gray fibers intergrown with chalcedony and opal in cracks in the lava flows. Moganite's physical and X-ray properties are almost identical to quartz.
Florke submitted a description of moganite to the International Commission on Mineral Names and Naming (I.C.M.N.N.), who disapproved of it. They felt that not enough evidence was presented to show that moganite was not simply a mixture of quartz and other minerals. Florke and his coworkers then committed a cardinal mineralogical sin by publishing their data, using the name moganite as though it had been approved. They published more articles on moganite over the next decade, proving it was a unique mineral. They never resubmitted their data to the I.C.M.N.N. The I.C.M.N.N. just as stubbornly refused to approve the name without such an official resubmission. So goes mineralogy politics!
A recent article by Peter Heaney and Jeffrey Post in Science added insult to injury. They reported finding moganite in nearly all of the 150 samples of fine-grained quartz they tested from the Smithsonian's mineral collection. Some samples contained over 75% moganite mixed with the quartz. Chert had the most moganite. Flint contained 13 -17% moganite. Agate had 5 to 20% moganite. Silicified corals from the Tampa Bay area in Florida averaged 20% moganite. The only samples found with no moganite were jaspers from iron formations, the weathered outer rinds of agates and Arkansas novaculite. Clearly moganite is a very overlooked mineral.
Why moganite forms is not known. In the Canary islands it forms under extremely dry surface conditions. Since moganite is more soluble than quartz, it leaches out of chalcedony, which could account for chalcedony's lower density and higher permeability.
Here is a mineral we all have that doesn't have an approved name. What is a conscientious mineral colle'ctor to do? We'd best just label things “quartz", "agate" or "chalcedony" until the semantic dust settles.
-Dr. Bill Cordua, University of Wisconsin-River Falls
References:
Florke, O.W., U. Florke and U. Geise, 1984, "Moganite: a new microcrystalline silica mineral", Nues Jahrb. Mineralogy, vol. 149, p. 325-336.
Heaney, Peter and Post, J.E., 1992, "The widespread distribution of a novel silica polymorph in microcrys alline quartz", Science, vol. 255, p.441-444.