Re: New Thread.


"Deirdre Sholto Douglas" <finch.enteract@xxxxxxx> wrote in message
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George wrote:

"Deirdre Sholto Douglas" <finch.enteract@xxxxxxx> wrote in message
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Madra Dubh wrote:

"George" <george@xxxxxxxxxxxxxxxxxxxxx> wrote in message
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http://www.youtube.com/watch?v=OU9N-ApYYJU

Now I've shown my ignorance.
I thought Dolomite and Limestone to be the same.
Are they not both mostly Calcium Carbonate?
What distinguishes one from the other>
(I was a spelunker of sorts in my youth, there being many caverns in
East
Tennessee)

Dolomite, as I recall, has a magnesium component
in the mix.

It's not the bulk components which are used to dis-
tinguish things, it's the minor ones. Hematite and
Lepidocrosite are both mostly iron, the difference
is how that iron is configured in the system.

Deirdre

That is not entirely correct. The major components of a rock are
primarily
what distinguishes one rock from another.

It depends, I should think, on the definition of
"rock"...my world doesn't have rocks, it has
solid phase and soluble compounds and when
solid, they're never more than a few microns
thick, hence I distinguish things by chemical
conformation. Solid phrase iron compounds
frequently have the same bulk components, it's
the way those components are configured and
the minor additions which makes them differ
from one another.

If one's entire world consists of a laboratory, I could see where this
might be the case. In the real world, geologists label rocks according to
their major constituents, those labels acquiring modifiers based on
secondary minerals. Hence, a Limestone with a high magnesium content might
be called "dolomitic limestone", etc.

Additionally, there's no "limestone", there are
only various polymorphs and flavours of calcium
carbonate...

Umm, incorrect, since most limestone has only a percentage of it's
composition as calcium carbonate. Other constituents include, dolomite,
quartz (in the form of sand or chert and other forms of microcrystalline
quartz/silica, phosphate nodules, mica, bitumen, and various clays, etc.).
There are several classifications of carbonate rocks in use, nearly all of
which still recognize the term "limestone".

some of the old timers, however,
still refer to CaMg(CO3)2 as "dolomite"...

Gee, I guess that puts about 99% of all geologists in the catagory of "old
timers", eh?

just as
they refer to Sodium Hydroxide as "caustic" and
copper sulphate as "blue vitriol". When I hear
"dolomite" it simply means they want a CaCO3
with a magnesium component.

No offense intended, but I'm glad you never worked in any labs where my
samples were analyzed. Fact: Dolomite is not simly "CaCO3 with a
magnesium component." It is a completely different mineral from calcite,
with a different atomic structure, and a different chemical, crystalline
and other physical properties, and has been recognized as such
internationally for over 150 years. Compare and contrast the following:

CaCO3 - http://webmineral.com/data/Calcite.shtml
-----
Molecular Weight = 100.09 gm
Calcium 40.04 % Ca 56.03 % CaO
Carbon 12.00 % C 43.97 % CO2
Oxygen 47.96 % O
______ ______
100.00 % 100.00 % = TOTAL OXIDE
Empirical Formula: CaCO3
Environment: Found in sedimentary, igneous, and metamorphic rocks.
IMA Status: Valid Species (Pre-IMA) 1845
Locality: Common world wide. Link to MinDat.org Location Data.
Name Origin: From the Latin, calx, meaning lime.
Synonym: Glendonite - pseudomorph
ICSD 73446
Manganocalcite - variety
Nicols
PDF 5-586
Travertine

Calcite Crystallography
Axial Ratios: a:c = 1:3.41992
Cell Dimensions: a = 4.989, c = 17.062, Z = 6; V = 367.78 Den(Calc)= 2.71
Crystal System: Trigonal - Hexagonal ScalenohedralH-M Symbol (3 2/m) Space
Group: R 3c
X Ray Diffraction: By Intensity(I/Io): 3.035(1), 2.095(0.18), 2.285(0.18),

Physical Properties of Calcite
Cleavage: [1011] Perfect, [1011] Perfect, [1011] Perfect
Color: Colorless, White, Pink, Yellow, Brown.
Density: 2.71
Diaphaniety: Transparent to translucent to opaque
Fracture: Brittle - Conchoidal - Very brittle fracture producing small,
conchoidal fragments.
Habit: Crystalline - Coarse - Occurs as well-formed coarse sized crystals.
Habit: Massive - Uniformly indistinguishable crystals forming large masses.
Habit: Stalactitic - Shaped like pendant columns as stalactites or
stalagmites (e.g. calcite).
Hardness: 3 - Calcite
Luminescence: Fluorescent.
Luster: Vitreous (Glassy)
Streak: white

Dolomite - http://webmineral.com/data/Dolomite.shtml
--------
CaMg(CO3)2
Composition: Molecular Weight = 184.40 gm
Calcium 21.73 % Ca 30.41 % CaO
Magnesium 13.18 % Mg 21.86 % MgO
Carbon 13.03 % C 47.73 % CO2
Oxygen 52.06 % O
______ ______
100.00 % 100.00 % = TOTAL OXIDE
Empirical Formula: CaMg(CO3)2
IMA Status: Valid Species (Pre-IMA) 1791
Locality: Link to MinDat.org Location Data.
Name Origin: Named after the French mineralogist and geologist, Deodat Guy
Tancrede Gratet de Dolomieu (1750-1801).
Synonym: ICSD 31336
PDF 36-426

Dolomite Crystallography
Axial Ratios: a:c = 1:3.29409
Cell Dimensions: a = 4.842, c = 15.95, Z = 3; V = 323.85 Den(Calc)= 2.84
Crystal System: Trigonal - RhombohedralH-M Symbol ( 3) Space Group: R 3
X Ray Diffraction: By Intensity(I/Io): 2.883(1), 1.785(0.6), 2.191(0.5),

Physical Properties of Dolomite
Cleavage: [1011] Perfect, [1011] Perfect, [1011] Perfect
Color: White, Gray, Reddish white, Brownish white, Gray.
Density: 2.8 - 2.9, Average = 2.84
Diaphaniety: Transparent to translucent
Fracture: Brittle - Conchoidal - Very brittle fracture producing small,
conchoidal fragments.
Habit: Blocky - Rhombohedral - Crystal shape resembles rhomohedrons.
Habit: Crystalline - Coarse - Occurs as well-formed coarse sized crystals.
Habit: Massive - Uniformly indistinguishable crystals forming large masses.
Hardness: 3.5-4 - Copper Penny-Fluorite
Luminescence: None.
Luster: Vitreous (Glassy)
Streak: white

I've got a lot more data to compare and contrast if this doesn't convince
you.

<shrug> It's in a different bottle on the same
shelf.

<Shrug> Are you surprised?

For instance, if there is more
calcite than dolomite in a sedimentary rock (carbonates being the
primary
constituent), the rock is called limestone. If the predominant mineral
is
dolomite, it is called dolomite, or more properly, dolostone.

You world obviously contain Big Things which can
be held in the hand, mine does not...your rock is
my e- acceptor/buffering agent...although I try
to avoid using carbonates and/or phosphates ex-
cept as needed for metabolism...a major pain,
they are, always precipitating when one doesn't
want them to and filtre sterilisation is a nuisance.
A right mess when uranyls enter the picture as
well, makes it hard to tell who's doing what.

Given that rather major difference in perspective,
it's not surprising we view the same things from
different angles.

Deirdre

Indeed. One of my favorite professors was an old chemist who hated
geologists (but liked me for some odd reason). I am correct, though.
Calcite and dolomite are separate and valid mineral species, and have been
recognized as such for a very long time. I suspect that you spend most of
your time, as you say, analyzing minute quantities of samples, are aren't
generally concerned with the intricacies of how and what various minerals
form rocks. That's where I come in. At any rate, limestone is more
soluble than dolomite/dolostone, and so there are many more caves formed in
limestone than dolomite/dolostone.

George


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Relevant Pages

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  • Re: Quartz Origins (Uh-Oh, Another Geology Post)
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