Re: Daryl Krupa: Geology Question (KRS related)

Steve Marcus wrote:
<snip>
> What I've learned, so far, about
> the geology of the dating method employed
> is that a weathering comparison was used to date the inscription
> by comparing the KRS (which is
> greywacke, a meta-sedimentary rock

Steve:
Just a point of clarification:
graywacke (American spelling) is a sedimentary rock;
metagraywacke is a metamorphic rock,
which could be called meta-sedimentary.
The KRS is best described as metagraywacke,
in that it is graywacke that shows signs of
having undergone metamorphosis
(i.e., changes resulting from heating, pressing,
and crystal growth and substitution, etc.).
What does Wolter call it?

> bearing these micas: biotite, chlorite and muscovite)

Chlorite is a flat *** silicate, like the micas,
but it is not generally called a mica, though some would see it as
related. From:

http://www.eos.ubc.ca/personal/groat/claymanual.htm

"Chlorite may be seen as a regular alternation of mica and brucite
layers."

Also:

http://www.glossary.oilfield.slb.com/Display.cfm?Term=mica

See Table I here:

http://www.minsocam.org/msa/collectors_corner/arc/nomenclaturecl1.htm

Also useful:

http://mineral.galleries.com/minerals/silicate/class.htm#phyllo

See also this description of the metamorphic
origins of mica and chlorite:

http://csmres.jmu.edu/geollab/Fichter/MetaRx/Metatexture.html

Does Wolter say that chlorite is a type of mica?

> with three slate (a metamorphic rock) tombstones
> from Augusta Maine
> that were selected from a much larger number of tombstones
> because the mica grain size in samples of those three tombstones
> was comparable to the grain size of the KRS micas.
> The authors only discuss biotite,
> leading me to believe that the tombstones
> did not have the other KRS micas.

I wouldn't go that far. Absent a detailed description
of the Augusta tombstones, we don't know that.

Did Wolter say which formation the tombstone slate
was quarried from?

Yet the specific type of biotite was not given.
I am not a petrology or mineralogy wizard, but
I have seen descriptions of a single rock type
with four phases of biotite, with different chemical
makeup in each.
The longer a rock is metamorphosed,
the more the biotite that it contains changes;
e.g., garnet can grow at the expense of biotite.
I have also seen analyses of biotite weathering that distinguished
between different types.
This might be a case of comparing apples and
oranges, WRT weathering characteristics of biotite.

> The tentative dating for the KRS is
> "older than 200 years" because
> all of the mica minerals on
> the "man-made surfaces" of the KRS
> have weathered away
> (it's not present in either the inscription or
> the surfaces bearing the inscription),
> while the biotite on the tombstones
> (average age 194 + or - 5 years)

Hmmm? I had thought that that was
about the age of the oldest tombstone sampled.

> is still present,
> although it is severly weathered.

If the weathering characteristics of the biotites
in the Augusta tombstones and the KRS were similar,
then that 200-year-plus age estimate might be
a fair assumption.
But it is as yet only an assumption, as
the congruency of the two materials' weathering
characteristics has not been proven.

> It is noted that the tombstones are from
> a geographical area stated in the text to
> average 17 inches of rain per year more than
> falls in Kensington (although tabular data
> in the book states that the average is
> 17 inches of *precipitation*,

A minor flaw, perhaps resulting from
an assumption that precipitation in Maine
is mainly plain rain.

> and although it is stated that both above and
> below grade samples should have been compared,
> only above-grade samples were collected from
> the tombstones because the ground was frozen
> and covered with a foot of snow.

If we knew the burial history of the KRS, and
how long any one side had buried, under what
conditions, and for what amount of time, that
might be a significant problem.
But we don't, so I am content to limit the
discussion to sub-aerial weathering (i.e.,
that which comes about during exposure to weather).

> Leaving aside the issue of differences in
> average temperatures between Augusta ME and
> Kensington, MN, as well as acid rain issues,
> I'm wondering whether the comparison made by
> the authors is geologically sound in the
> sense of comparing apples to apples.
> I don't know enough geology to know whether
> the fact that the two rocks are of different
> types, and the micas on the two different types
> of rocks also being different different
> (the rocks are reported as having only biotite
> in common) amounts to comparing apples to bananas.

I would be surprised to find that the only common
mica between the KRS and the Augusta tombstone slate
was biotite.
A mention of the tombstones' parent formation
would have been useful in that regard, or even
the location of the quarry or quarries that
supplied the cemetary.

From an ancient tome on American slate quarrying,
WRT Maine slate:

1906
Slate Deposits and Slate Industry of the United States

http://www.cagenweb.com/quarries/states/me-slate_1906.html

"
The constituents of this slate, arranged in
the order of their decreasing abundance,
appear to be muscovite (sericite), quartz,
chlorite, biotite, pyrite, carbonaceous or
graphitic matter, magnetite, rutile, and apatite.
"

Two types of mica there,
the most important constituent of the rock (i.e.,
the matrix that the other minerals occur within)
being
the mica known as muscovite.

Similarly from 1914:
Excerpts From
Slate in The United States

http://www.cagenweb.com/quarries/states/me-slate_1914.html

That Maine slate seems to be made mainly of
another type of mica than biotite seems
to have been known for a century. Or more.

> Any light you could shed on these points
> would be appreciated.

Hoping that this has been a start on answering,
Daryl Krupa

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