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Roger Warin
Joined: 23 Jan 2013
Posts: 1231
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 Posted: May 30, 2013 15:58 Post subject: Pyrrhotite - I lost its origin |
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Hello,
I lost the label of this pyrrhotite is lying in my collection for 38 years ...
Can you find its origin? Maybe Mexico.
Thank you.
Roger.
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Jordi Fabre
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Peter Megaw
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Joined: 13 Jan 2007
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| Posted: May 30, 2013 18:28 Post subject: Re: Pyrrhotite - I lost its origin |
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Jordi's right ...Defitinely West Camp of Santa Eulalia...99% probable from the Potosi Mine.
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Roger Warin
Joined: 23 Jan 2013
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| Posted: May 31, 2013 00:50 Post subject: Re: Pyrrhotite - I lost its origin |
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Hello Jordi, Peter
Thank you very much. I needed to know the origin because I compare pyrrhotite with troilite (FeS) that it comes from space.
Pyrrhotite is magnetic, while troilite is not magnetic. This is due to deficiencies in iron in the pyrrhotite.
Roger.
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Carles Curto
Joined: 14 Sep 2006
Posts: 160
Location: Barcelona
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| Posted: May 31, 2013 03:41 Post subject: Re: Pyrrhotite - I lost its origin |
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| I'm also agree with Jordi and Peter due to the Quartz or Calcite coating, to the deep greenish-brass color and the crystal shape, that is usually prismatic (sometimes very elongated) or very thick tabular on Santa Eulalia samples. It is rare thin tabular or laminar.
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Jordi Fabre
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| Posted: Jun 01, 2013 06:48 Post subject: Re: Pyrrhotite - I lost its origin |
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Due the great quality of the Peter's answer I copied it in the Featured Columns of FMF section, in this thread: Practical Mineralogy #2: What pseudos tell us about deposit genesis - (12) in order to save it and preserve it.
Below is the Peter's answer:
| Peter Megaw wrote: | | In https://www.mineral-forum.com/message-board/viewtopic.php?p=31794#31794 Carles Curto wrote: | | I'm also agree with Jordi and Peter due to the Quartz or Calcite coating, to the deep greenish-brass color and the crystal shape, that is usually prismatic (sometimes very elongated) or very thick tabular on Santa Eulalia samples. It is rare thin tabular or laminar. |
Carles...to clarify a bit. Thin platy and laminar pyrrhotite is actually quite common at Santa Eulalia, especially in the San Antonio Mine of the East Camp...and .why we don't see it is explained below.
Overall the Santa Eulalia district shows a very well developed zonation of iron-sulfide species. Pyrrhotite is overwhelmingly dominant at depth, typically as stout prismatic crystals with strong magnetism. Pyrite dominates in the upper parts of the system...actually in areas where it was thoroughly oxidized so we don't see much crystalline material preserved...but there is enough to see that there is NO pyrrhotite up high. The intermediate zone shows a progressive upward replacement of early formed pyrrhotite by pyrite (I have discussed the genetic implications of this sulfidation reaction before). Up high the replacement is complete but the degree of replacement diminishes to depth. In effect there is a continuous upwards transition from deep pyrrhotite to pyrite after pyrrhotite to pyrite.
This transition is seen in many systems (Dalnegorsk) an in some related systems...like the enormous San Martin-Sabinas skarn system of Zacatecas, the transition continues to depth as a change from magnetic to non-magnetic pyrrhotite. (This makes it very hard to separate from the desired ore minerals and dramatically reduces the economic viability of the deepest ores in the mine)
Returning to Santa Eulalia
In the San Antonio Mine there is a pronounced development of platy pyrrhotite in the upper half of the transition zone...levels 7-10 Platy pyrrhotite is also present in the West Camp, but athough it does appear to be more abundant in the upper part of the transition zone it is harder to define particular zoning largely because of the highly variable depth of oxidation in the West Camp...which can vary 100-400m in elevation over very short lateral distances
In both mines the zones of significant development of platy pyrrhotite are overlapped in part by oxidation. Pyrrhotite is fragile enough when "healthy" but when partially oxidized it decrepitates quickly. (This is equally damaging to prismatic pyrrhotite). Further, the partially pseudomorphed pyrrhotites seem to be unstable even when not oxidized...so they tend also to be crumbly and unstable. The situation in the San Antonio Mine is doubly bad because the oxidation level (which is almost complete to the 8th level with a flat bottom marking the water table) coincides closely with the transition zone and the zone of well developed platy pyrrhotite. I have opened pockets in that part of the mine with dinner plate sized platy pyrrhotites, partially replaced by pyrite and partially oxidized, that crumbled to piles of sulfurous fragments with a single touch. West Camp preservation is much more variable because of the variable oxidation, so transition zone materials are preserved in some places and not in others.
I have long been struck by the similarity of the Santa Eulalia pyrite replacements after platy pyrrhotite to those from Dalnegorsk...and wondered if the samples showing partial replacement are stable long-term or not. It's been 20 years since these flooded the market here and I would be interested to hear about collector's experiences. |
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Peter Megaw
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Location: Tucson, Arizona
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| Posted: Jun 01, 2013 08:37 Post subject: Re: Pyrrhotite - I lost its origin |
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Thanks Jordi
Antonio et al...once pyrrhotite gets to swelling and cracking it is beyond hope...get rid of it before it infects other pieces!
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Antonio Alcaide
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Joined: 23 Aug 2009
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| Posted: Jun 01, 2013 09:18 Post subject: Re: Pyrrhotite - I lost its origin |
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| Peter Megaw wrote: | | Antonio et al...once pyrrhotite gets to swelling and cracking it is beyond hope...get rid of it before it infects other pieces! |
Thanks again. I will do it inmediately.
Regards
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Duncan Miller
Joined: 25 Apr 2009
Posts: 138
Location: South Africa
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| Posted: Jun 01, 2013 11:39 Post subject: Re: Pyrrhotite - I lost its origins |
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They 'infect' each other? Is the decay biologically mediated? What is the process? (Or were you joking?)
Duncan
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Peter Megaw
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Joined: 13 Jan 2007
Posts: 973
Location: Tucson, Arizona
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| Posted: Jun 01, 2013 13:15 Post subject: Re: Pyrrhotite - I lost its origin |
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Only half joking...the sulfurous vapors generated through oxidation of the pyrrhotite will chemically attack other sulfides they are in atmospheric contact with...especially if closed up together in a drawer or box. This is true of any "rotting" sulfide...pyrite and marcasite being equally if not more susceptible than pyrrhotite.
However...given what we have learned over the last 30 years about sulfur-powered microorganisms and the role some play in depositing sulfides...it is quite likely that bacteria play a role in the decomposition of sulfides too.
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