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Noob....need a little guidance here...
  
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MedIKate




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PostPosted: Oct 30, 2018 01:39    Post subject: Noob....need a little guidance here...  

All I know or at least think I know is that these are agates..... Can someone point me into the direction of more info? Like anything relevant that seems itself apart from another or what classifies it? I really got nothing...


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Tobi




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PostPosted: Oct 30, 2018 03:09    Post subject: Re: Noob....need a little guidance here...  

These are geodes, the shell is agate and the crystals inside are amethyst, these specimens are most likely from Brazil ...
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Bob Harman




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PostPosted: Oct 30, 2018 04:37    Post subject: Re: Noob....need a little guidance here...  

You have MEXICAN (sorry TOBI they are not from Brazil!) Las Choyas geodes from Chihuahua State. They are often called coconut geodes as their outer rind vaguely resembles a coconut rind. Within the central cavity a light purple amethyst quartz is present. They are NOT considered agates. Nice examples. These are commonly available on auction sites such as eBay. BOB
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Bob Harman




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PostPosted: Oct 30, 2018 05:14    Post subject: Re: Noob....need a little guidance here...  

This might be a good time to discuss the similarities and differences between an AGATE and a GEODE.

The AGATE and GEODE are close cousins, but for classification, are usually considered separately. Both are rock structures usually composed of microcrystalline quartz with a druzy lining of quartz crystal tips in a hollow cavity, if one is present. Calcite geodes and celestine geodes (Madagascar) also occur. An AGATE has banded or lacy microcrystalline quartz patterns of differing colors. It is most often solid, or if there is a central cavity, it is small. Agates are most often found where other agates are found and often in igneous environments.
GEODES, on the other hand, are often hollow with a larger cavity. They can, however, be solid, but their rind doesn't usually show a prominent banding pattern. Geodes often contain secondary minerals along with the druzy quartz in their central cavity. Geodes are found both in igneous and sedimentary environments and usually where other geodes are found.
Agates and geodes usually are not found side by side in the same locations; more commonly it is one or the other at the individual location.
It is important to state that there are occasional intermediate forms between the two, making classification difficult, however the Mexican forms as seen in this posting, are considered geodes as the banding patterns in their rind is not prominent and their cavities are large and prominent.
BOB
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Tobi




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PostPosted: Oct 30, 2018 05:43    Post subject: Re: Noob....need a little guidance here...  

Bob, maybe we have different understandings of the term "geode", so I don't get your point:

I did not mix up the terms agate (which is a variety of chalcedony and thus a mineral species name) and geode (which means a hollow rock structure and thus refers to a certain form), this is like explaining the "difference" between a cow and a four-legged animal ;-)

I don't understand what you mean with "intermediate forms" between these two completely different concepts. An agate can be part of a geode but can also occur in other forms. And a geode may contain agate, but is also a geode when it consists of other minerals.

And sorry not to agree in this point: You say these specimen are not considered agate, but what is the grey rind that surrounds them? I'm quite sure this is agate (similiar to those from the typical Brazilian amethyst geodes) ...

Regards,
Tobi

Bob Harman wrote:
You have MEXICAN (sorry TOBI they are not from Brazil!) Las Choyas geodes from Chihuahua State. They are often called coconut geodes as their outer rind vaguely resembles a coconut rind. Within the central cavity a light purple amethyst quartz is present. They are NOT considered agates. Nice examples. These are commonly available on auction sites such as eBay.
This might be a good time to discuss the similarities and differences between an AGATE and a GEODE.
The AGATE and GEODE are close cousins, but for classification, are usually considered separately. Both are rock structures usually composed of microcrystalline quartz with a druzy lining of quartz crystal tips in a hollow cavity, if one is present. Calcite geodes and celestine geodes (Madagascar) also occur. An AGATE has banded or lacy microcrystalline quartz patterns of differing colors. It is most often solid, or if there is a central cavity, it is small. Agates are most often found where other agates are found and often in igneous environments. GEODES, on the other hand, are often hollow with a larger cavity. They can, however, be solid, but their rind doesn't usually show a prominent banding pattern. Geodes often contain secondary minerals along with the druzy quartz in their central cavity. Geodes are found both in igneous and sedimentary environments and usually where other geodes are found. Agates and geodes usually are not found side by side in the same locations; more commonly it is one or the other at the individual location. It is important to state that there are occasional intermediate forms between the two, making classification difficult, however the Mexican forms as seen in this posting, are considered geodes as the banding patterns in their rind is not prominent and their cavities are large and prominent.
BOB
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Carles Millan
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PostPosted: Oct 30, 2018 05:52    Post subject: Re: Noob....need a little guidance here...  

Tobi wrote:
I did not mix up the terms agate (which is a variety of chalcedony and thus a mineral species name)...

Can't find the species chalcedony in the IMA list. Are you sure?
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PostPosted: Oct 30, 2018 06:05    Post subject: Re: Noob....need a little guidance here...  

Carles Millan wrote:
Tobi wrote:
I did not mix up the terms agate (which is a variety of chalcedony and thus a mineral species name)...
Can't find the species chalcedony in the IMA list. Are you sure?
At least "agate" is a mineral name and refers to a certain kind of microcrystalline quartz (no matter if the IMA lists it or not), it is not a structural description like "geode". Just wanted to point out the difference because I'm confused about Bob's post in which a certain name and a general description are mixed up ...
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PostPosted: Oct 30, 2018 09:02    Post subject: Re: Noob....need a little guidance here...  

Tobi wrote:
Bob, maybe we have different understandings of the term "geode", so I don't get your point:

I did not mix up the terms agate (which is a variety of chalcedony and thus a mineral species name) and geode (which means a hollow rock structure and thus refers to a certain form), this is like explaining the "difference" between a cow and a four-legged animal ;-)

I don't understand what you mean with "intermediate forms" between these two completely different concepts. An agate can be part of a geode but can also occur in other forms. And a geode may contain agate, but is also a geode when it consists of other minerals.

And sorry not to agree in this point: You say these specimen are not considered agate, but what is the grey rind that surrounds them? I'm quite sure this is agate (similiar to those from the typical Brazilian amethyst geodes) ...

Regards,
Tobi

Bob Harman wrote:
You have MEXICAN (sorry TOBI they are not from Brazil!) Las Choyas geodes from Chihuahua State. They are often called coconut geodes as their outer rind vaguely resembles a coconut rind. Within the central cavity a light purple amethyst quartz is present. They are NOT considered agates. Nice examples. These are commonly available on auction sites such as eBay.
This might be a good time to discuss the similarities and differences between an AGATE and a GEODE.
The AGATE and GEODE are close cousins, but for classification, are usually considered separately. Both are rock structures usually composed of microcrystalline quartz with a druzy lining of quartz crystal tips in a hollow cavity, if one is present. Calcite geodes and celestine geodes (Madagascar) also occur. An AGATE has banded or lacy microcrystalline quartz patterns of differing colors. It is most often solid, or if there is a central cavity, it is small. Agates are most often found where other agates are found and often in igneous environments. GEODES, on the other hand, are often hollow with a larger cavity. They can, however, be solid, but their rind doesn't usually show a prominent banding pattern. Geodes often contain secondary minerals along with the druzy quartz in their central cavity. Geodes are found both in igneous and sedimentary environments and usually where other geodes are found. Agates and geodes usually are not found side by side in the same locations; more commonly it is one or the other at the individual location. It is important to state that there are occasional intermediate forms between the two, making classification difficult, however the Mexican forms as seen in this posting, are considered geodes as the banding patterns in their rind is not prominent and their cavities are large and prominent.
BOB


I think from my reading my two learned friends here (Bob and Tobi) are pretty much in agreement, although Tobi points out the important distinction that "geode" is a descriptive term and "Agate" is a mineralogical term for a member of the microcrystalline quartz family.
Having said that, I disagree with Tobi that in this particular case the "rind" can be classified as an agate. It shows very little sign of being deposited in a coherent way with distinct banding other than a millimetre or two at the base of the crystalline layer in the larger geode that I think could be the beginnings of macrocrystal growth. The smaller geode has no sign even of that. I would interpret the majority of the rind to be essentially an alteration product of the original matrix, which has been cemented by silica presumably in the very early stages of stabilization of the cavity. I would therefore join the side of the Geodesists here, rather than the Agatomanes.

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Susan Robinson




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PostPosted: Oct 30, 2018 09:57    Post subject: Re: Noob....need a little guidance here...  

Chalcedony is a form of quartz, as are agates, jasper, aventurine, amethyst, citrine, smoky quartz, and even tiger's eye, which is a quartz replacement of crocidolite. Did I forget any?
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PostPosted: Oct 30, 2018 10:42    Post subject: Re: Noob....need a little guidance here...  

Not sure about this and it may be semantics, but there is (to me) a big difference if a field collector comes to you and says "I found an agate" or if the collector says "I found agate".

One statement (again to me) implies a rock structure while the other statement implies knowledge of the microcrystalline quartz makeup of the find.

If you agree with this interpretation of "an agate" vs "agate", then my original posting seems (again to me) seems correct. YES?? NO??

Anyway the shown Mexican examples have always been referred to as geodes because they have a large cavity, relatively little banding in their rinds and often contain secondary minerals in the cavity.
BOB
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PostPosted: Oct 30, 2018 10:53    Post subject: Re: Noob....need a little guidance here...  

Hi Bob,

after reading your last post, I think we maybe had a linguistic problem, not a mineralogical one ;-) I didn't know that (in English) there's a difference between "agate" and "an agate". I assumed that there was only one meaning and that "agate" always refers to microcrystalline quartz, didn't know there is a colloquial meaning that refers to a geode ...

I took my native language as a basis where "Achat" (agate) always means the microcrystalline quartz and "Geode" means a hollow rock structure filled with crystals ...

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Tobi
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Dale Hallmark




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PostPosted: Oct 30, 2018 11:02    Post subject: Re: Noob....need a little guidance here...  

I'm an American and English speaker and I don't see any difference between agate and an agate.

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PostPosted: Oct 31, 2018 10:13    Post subject: Re: Noob....need a little guidance here...  

Agate is a noun so I do not see any difference. All I can think of is that maybe, like 'sheep', it could be both a singular noun or a plural one and 'I found agate' is is the plural noun form?
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PostPosted: Oct 31, 2018 11:14    Post subject: Re: Noob....need a little guidance here...  

Coming back to the original question, I think that a proper name for these samples could be underdevelloped agate
To me, all agates are geodes (or were in a primary state, later collapsed) but not all geodes are agate...
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PostPosted: Oct 31, 2018 16:46    Post subject: Re: Noob....need a little guidance here...  

Josele wrote:
Coming back to the original question, I think that a proper name for these samples could be underdevelloped agate
To me, all agates are geodes (or were in a primary state, later collapsed) but not all geodes are agate...


I have to disagree with Josele, not all agates are geodes.
The first picture show a slice of a so-called "blue lace agate", it can be seen that the sample, defined by its upper and lower boundaries, is quite flat. The second picture is of a thin (+/- 1 cm) agate self-collected in a rock fissure, also flat.

I do concur with Tobi in the sense that "geode" is a morphological term.



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PostPosted: Oct 31, 2018 20:43    Post subject: Re: Noob....need a little guidance here...  

Vinoterapia wrote:
I have to disagree with Josele, not all agates are geodes.
...
I do concur with Tobi in the sense that "geode" is a morphological term.
Touché!
...Or maybe your flat agate is the exception that confirms the rule? ;-)

I also understand geode as a morphological / structural term.

Regarding semantics, as language cognate curiosity, let me note that in English geode implies a rounded shape but in Spanish refers to any kind of vug filled of crystalline matter, as is yet noted in Mindat Agate Page (last paragraph) and is defined by the Real Academia Española. I don't know if in others latin-root languages geode implies or not a rounded shape.
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PostPosted: Nov 01, 2018 03:06    Post subject: Re: Noob....need a little guidance here...  

I have always translated 'geoda' as either 'geode' or 'pocket' and I work out which to use from the context. Checking on Linguee (the site the professional translators I know prefer, as it gives examples) I see that most people translate 'geode'->'geoda' while some do 'geoda'->'geode' while others do 'geoda'->'pocket'. It all depends on the context, but the simple cognate is not perfect both ways.

Maybe 'geode' is 'geoda redonda', which appears in some Linguee examples.

Weirdly, I would never use 'geoda'->'vug' but that may be a personal preference. I see 'cyst' has also been used for 'geoda' in Linguee
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