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I show you the back of the cabochon made of white opal. The dendrite is all inside. This is the drawer where it has been for 13 years. The only change was the addition of the azurite and malachite sam ...

Years ago I cut a beautiful clear opal cabochon with a beautiful well-centered black dendrite. The best quality of that cabochon was that it was from Cabo de Gata, Almeria, Spain a little-known locati ...

Sorry Alfredo,
my comment comes from the fact that gypsum does not appear on the mindat page. Maybe you can add it.

Hi Alfredo,
perhaps you are referring to the gypsum of Mina Rica, Pulpì, Almeria, Spain, where gypsum crystals show inclusions of celestine needles and white spherulites. In my opinion, in that case ...

Hi Daniel,
From the hand sample the chalcedony is spherical and I don't see any faces. From the last photo you can see chalcedony filling a cavity inside a gypsum crystal, but I still don't see any f ...

I don't see replacing after gypsum. The formation of your spherulites looks like the growth of a concretion. There is a granular gypsum with porosity. A Si-rich solution flows through the porosity. Th ...

My impression is that it is all chalcedony. Chalcedony is a fibrous silica polymer so the structure you see in thin section looks very similar to it. Cristobalite is stable over 1470ºC, Therefore it i ...

Cristobalite is high T mineral. All supposed cristobalite spherulites are other silica phase.

Dear Herwig,
I like to write about debatable things in my posts so I really appreciate your questions.
1) "why do you think it is monocrystalline? There is no way for you to see that with the n ...

I am reopening this interesting topic because I came across a bivalve in fluorite from Belgium that seems very unique to me as it appears to be made of monocrystalline fluorite.
I find the discussion ...

I don't understand why you say that LDG was formed during a comet impact, but then you say that it is not a tektite. Considering that the other tektites are also the result of comet impacts. Please de ...

Yes Alfredo,
Obsidians often have a water content below 0.1%, compared to at least about 2% of a rhyolite which can be up to 7%. Tektites are almost anhydrous, <0.02%. The LDG is a somewhat anomal ...

Hello Roger,
I was in the Tunguska Italian team of 1991 with the physicists from Bologna and I was the only geologist, in charge of distinguishing the particles of volcanic origin from those potentia ...

Yes it is a typical pseudomorph of calcite after aragonite. The locality is Agrigento, probably from the Cozzo Disi Mine.

Hi Roger, beautiful photos and interesting discussion. I am always happy to discuss with you. I would like to clarify some points on which I have studied for a long time especially on the origin of sp ...

Author	Message
Topic: Ghost dendrite
marco campos-venuti Replies: 7 Views: 1831	Forum: Minerals and Mineralogy Posted: May 03, 2025 04:17 Subject: Re: Ghost dendrite
marco campos-venuti Replies: 7 Views: 1831	I show you the back of the cabochon made of white opal. The dendrite is all inside. This is the drawer where it has been for 13 years. The only change was the addition of the azurite and malachite sam ...
Topic: Ghost dendrite
marco campos-venuti Replies: 7 Views: 1831	Forum: Minerals and Mineralogy Posted: May 02, 2025 02:20 Subject: Ghost dendrite
marco campos-venuti Replies: 7 Views: 1831	Years ago I cut a beautiful clear opal cabochon with a beautiful well-centered black dendrite. The best quality of that cabochon was that it was from Cabo de Gata, Almeria, Spain a little-known locati ...
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 09, 2025 13:24 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Sorry Alfredo, my comment comes from the fact that gypsum does not appear on the mindat page. Maybe you can add it.
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 09, 2025 06:32 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Hi Alfredo, perhaps you are referring to the gypsum of Mina Rica, Pulpì, Almeria, Spain, where gypsum crystals show inclusions of celestine needles and white spherulites. In my opinion, in that case ...
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 08, 2025 10:16 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Hi Daniel, From the hand sample the chalcedony is spherical and I don't see any faces. From the last photo you can see chalcedony filling a cavity inside a gypsum crystal, but I still don't see any f ...
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 08, 2025 07:59 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	I don't see replacing after gypsum. The formation of your spherulites looks like the growth of a concretion. There is a granular gypsum with porosity. A Si-rich solution flows through the porosity. Th ...
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 08, 2025 05:38 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	My impression is that it is all chalcedony. Chalcedony is a fibrous silica polymer so the structure you see in thin section looks very similar to it. Cristobalite is stable over 1470ºC, Therefore it i ...
Topic: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Forum: Minerals and Mineralogy Posted: Apr 08, 2025 03:40 Subject: Re: cristobalite vs chalcedony in thin section
marco campos-venuti Replies: 16 Views: 20786	Cristobalite is high T mineral. All supposed cristobalite spherulites are other silica phase.
Topic: When can a fossil be also considered a mineral?
marco campos-venuti Replies: 55 Views: 170821	Forum: The Ten Thousand Club Posted: Mar 25, 2025 07:21 Subject: Re: When can a fossil be also considered a mineral?
marco campos-venuti Replies: 55 Views: 170821	Dear Herwig, I like to write about debatable things in my posts so I really appreciate your questions. 1) "why do you think it is monocrystalline? There is no way for you to see that with the n ...
Topic: When can a fossil be also considered a mineral?
marco campos-venuti Replies: 55 Views: 170821	Forum: The Ten Thousand Club Posted: Mar 24, 2025 12:00 Subject: Re: When can a fossil be also considered a mineral?
marco campos-venuti Replies: 55 Views: 170821	I am reopening this interesting topic because I came across a bivalve in fluorite from Belgium that seems very unique to me as it appears to be made of monocrystalline fluorite. I find the discussion ...
Topic: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Forum: Minerals and Mineralogy Posted: Mar 19, 2025 11:27 Subject: Re: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	I don't understand why you say that LDG was formed during a comet impact, but then you say that it is not a tektite. Considering that the other tektites are also the result of comet impacts. Please de ...
Topic: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Forum: Minerals and Mineralogy Posted: Mar 18, 2025 03:10 Subject: Re: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Yes Alfredo, Obsidians often have a water content below 0.1%, compared to at least about 2% of a rhyolite which can be up to 7%. Tektites are almost anhydrous, <0.02%. The LDG is a somewhat anomal ...
Topic: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Forum: Minerals and Mineralogy Posted: Mar 17, 2025 15:29 Subject: Re: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Hello Roger, I was in the Tunguska Italian team of 1991 with the physicists from Bologna and I was the only geologist, in charge of distinguishing the particles of volcanic origin from those potentia ...
Topic: Calcite pseudomorph after aragonite from Sicily
marco campos-venuti Replies: 2 Views: 1809	Forum: FOR BEGINNERS: What is it? Where is it from? Posted: Feb 20, 2025 00:58 Subject: Re: Calcite pseudomorph after aragonite from Sicily
marco campos-venuti Replies: 2 Views: 1809	Yes it is a typical pseudomorph of calcite after aragonite. The locality is Agrigento, probably from the Cozzo Disi Mine.
Topic: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Forum: Minerals and Mineralogy Posted: Jan 14, 2025 17:06 Subject: Re: Libyan Desert Glass Structure
marco campos-venuti Replies: 45 Views: 88895	Hi Roger, beautiful photos and interesting discussion. I am always happy to discuss with you. I would like to clarify some points on which I have studied for a long time especially on the origin of sp ...