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Tschermakite series

Posted by Olav Revheim  
Tschermakite series
February 17, 2011 08:38AM
    
First Draft



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Can you help make this a better article? What good localities have we missed? Can you supply pictures of better specimens than those we show here? Can you give us more and better information about the specimens from these localities? Can you supply better geological or historical information on these localities?

The Tschermakite series minerals are minerals in the Amphibole group, see Amphibole Group main article for an overview of the group. The series contains the following minerals:

Aluminotschermakite
☐Ca2(Mg3Al2)Si6Al2O22OH2

Alumino-ferrotschermakite
☐Ca2(Fe3Al2)Si6Al2O22OH2

Ferri-Ferrotschermakite
☐Ca2(Fe3Fe2)Si6Al2O22OH2

Ferritschermakite
☐Ca2(Mg3Fe2)Si6Al2O22OH2

Ferrotschermakite
☐Ca2(Fe3FeAl)Si6Al2O22OH2

Tschermakite
☐Ca2(Mg3FeAl)Si6Al2O22OH2

The tschermakite-series of minerals is a series of rockforming minerals not known in specimens attractive to collectors. Tschermakite can be found in a wide range of geological environments, and in the 44 entries in Mindat includes calc-sislicate rocks, sulphide ores, gabbroes, vulcanic rocks as well as 3 meteorites. An extended literature search in petrografic and mineralogical literature would undoubtedly yield several tschermakite locations not currently included in the Mindat database, but since these minerals are rock-forming minerals, not all of them would qualify as a mineral occurance from a collector's point of view.

The tschermakite-series is closely related to the hornblende-series and has a slightly higher Al/Si proposion as well as pargasite-series(which is richer in Na).

The various Tschermakite-series minerals are identified on the relative distribution of Mg, Fe2+, Fe3+ and Al in the C position. Tschermakite itself probably is relative common, but is not often identified. This has to do with the cost and complexity in making a positive identification, and that accurate identification of the specimen has limited value for petrologists or ore-geologists. Also, the chemical environments in which Tschermakite is found are normally not that special or sexy, thus reducing the interest to invest time and money to identify the mineral. For mineral collectors, the cost of having the mineral identified will always exceed the value of the mineral specimen.

Some of the other end members in the series such as alumino-ferrotschermakite, ferritschermakite and ferri-ferrotschermakite will require rather special environments to form, and are as pr. 2008 hypotethical end members of the series, not described from nature. See Minerals not found in nature- Mindat message board

Tschermakite series minerals will not normally be collected by other collectors than the most enthusiastic systematic collectors, whether individuals or museums. Specimens will not be expensive unless some hitherto great new locations are discovered.


Pargasite
Tanzania
Kilimanjaro Region, Mt. Kilimanjaro, Longido

8,7 cm crystal Pargasite © Christian Bracke
4 cm FOV. Pargasite © JGW
.
4,7x4,5 cm Pargasite © Sobolewski

Both Aluminotschermakite and tschermakite are reported as occuring in the anyolite mines of Longido, Mt Kilimanjaro, Kilimanjaro Region, Tanzania. Analytical data provided from multiple sources indicate that the amphibole is the much more common mineral pargasite. Most collectors are more familiar the red rubies embedded in the beautiful green zoisite that is the anyolite rock. The pargasite is the black specks disturbing the nice green colour. Mindat defines the rock anyoite as « ... a metamorphic rock consisting of a granular mass of bright green chrome-zoisite with minor black tschermakite (commonly both as grains a few mm diameter), containing conspicuous coarse porphyroblasts of corundum (var. ruby) which may be up to several centimetres across.

As a generic petrographic description of this rock would be a rather clumsy mouthful, (something like "corundum-tschermakite zoisiteite" ) it has been given a more euphonious name descriptive of its striking colour in the language of the people living in the area of its type locality in Tanzania.». The rock name anyolite is derived from the word "Anyole" which is the Maasai word for green.

The zoisite/corundum/amphibole (anyolite) rock occurs in two outcrops, as veins no thicker than a few decimeters to a meter. It is believed that the two anyolite outcrops are connected as a single folded vein. The original mine consists of a ramp at 50 degrees and a dozen horizontal galleries operating through the same vein. The galleries are now in a pitiful state and the mine abandoned (as of 2000). Cuttings are not removed and landslides are frequent. The other outcrop ("Kijiji" ) was opened around 2000.

At the outcrops, mineralogical zonation is clearly visible;

1) serpentinized peridotite
2) A (phlogopite?) mica zone +/- gedrite, enclosing the ruby bearing amphibolite and anyolite
3) Dark amphibolite
4) Anyolite, constituting the core of the vein and with the following facies evident:
4a) massive anyolite with little or no amphibole
4b) "leopard" anyolite with black amphibole spots
4c) "zebra" anyolite with black amphibole stripes

Ruby ocurs as spots or anhedral crystals in thin hexagonal tablets. It is present in the anyolite as as well as in the amphibolite. The crystals can reach about fifteen centimeters in diameter.

The amphibole is generally considered as tschermakite both in Mindat and other mineralogical webpages as well as different dealers websites. The amphibole has also been named aluminotschermakite (Jochen Hintze (2010) Lapis, 35, #12, 15-19. ) but this identification is questioned due to lack of references with analytical data.

In his doctorial thesis: “Geology of Sapphire and Ruby Deposits -The example of the John Saul Ruby Mine, Mangare area, Southern Kenya” PhD Thesis, University of Nantes, Cedric Simonet, January 2000"Cedric Simonet gives a fairly detailed description of the ruby occurances at Longido including a series of chemical analysis' of the amphiboles.

He identifies three amphiboles in these rocks:
1) In the serpentinite host rock: Magnesiohornblende - 2 microprobed samples giving the following normalized formula (Na0,34,K0,04)0,38Ca1,82(Mg4,34,Fe0,45)4,89 (Si7,17Al1,13)8,3O22(OH)2 (average of 2 samples)
2) In the mica layer: An orthoamphibole optically indentified as “gedrite” occurs in the mica zone, probably as an alteration product of clinoamphiboles.
3) In the anyolite vein: 4 microprobed samples giving the following normalized formula: (Na0,71,K0,06,Ca0,05)0,82 (Ca1,87, Mg0,12, Mn0,01)2,00((Mg2,91,Fe0,87Ti0,01,Cr0,06,Al1,15)5,00 (Si5,89Al1,11)8,00O22(OH)2.

The amphibole from the anyolite analysed by Dr. Simonet contains Si=5,89, Mg/(Mg+Fe) =0,91 and (Na,K)>0,5. According to the identification tables listed in Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, commission on new mineral names (Leake et al. 1998). this amphibole classifies as pargasite, not tschermakite.

The amphibole in the anyolite rock is consequently known under three 3 different names, but only an ID as pargasite can be supported by analytical data.


Ferri-Ferrotschermakite
Sweden
Kopparberg, Ljusnarsberg, Västmanland Kaveltorp

4 cm sample Ferri-Ferrotschermakite? © 2008, JGW


Kaveltorp is one of the many metamorphic sulphide ores in the vicinity of Kopparberg ( the name meaning Mountain of Copper). The document «Iakttagelser angående mineralens paragenes och successionen i Kaveltorp. (With an English summary)» by Magnusson, N.H.(1930) does assumingly provide a good reference to the mineralogy in the mine. Unfortunately, I do not have access to this paper. Magnussons paper was written pre-IMA amphibole nomenclature, and consequently before the current definition of ferri-ferrotschermakite. It is therefore unlikely that his paper will shed sufficiently detailed light on the composition of the amphibole from Kaveltorp.


Ferri-ferro tschermakite with it's current definition l is a theoretical end-member not yet described from nature. It is therefore rather unlikely that this specimen is correctly identified, and it is also unlikely that ferri-ferrotschermakite is found here or anywhere else for that matter.

Ferrotschermakite
Australia
Victoria, Dookie, Dookie Mineralogical Reserve quarry Fast Navigation for finished Best Minerals articles.

Ferrotschermakite, 6mm FOV © Judy Rowe


Reference: Museum Victoria Collection.


Tschermakite
Italy
Campania, Naples Province., Somma-Vesuvius Complex, Monte Somma, San Vito, Ercolano, San Vito quarry

Tschermakite, 1,2 mm FOV © luigi chiappino


The San Vito Quarry is an abandoned pumice quarry that has been very rich in minerals. It is a part of the general Somma-Vesuvius area which is a composite central volcano composed of an ancient stratovolcano, Mount Somma, and more recently by a cone, the Vesuvius. The age of the oldest products in outcrop is about 25,000 years.The latest round of activity seems to have ended with the eruption of March 1944. This eruption was the beginning of a resting phase characterized by modest signs of seismic activity and fumarole (Arno et al., 1987).
More than 230 species has been found in this area, and is one of the most interesting places in Europe. There are in principle four different mineral forming environments, each with a different mineral assemblage;
I. Minerals that are found in the ejected limestone blocks of Monte Somma.2
II. Pneumatolytic minerals formed in cavities of leucotephrites and conglomeratic blocks ejected by Monte Somma and Vesuvius, or coating the walls of ancient lavas.
III. Fumarolic products.
IV. Minerals that occur as rock constituents of Vesuvius and Monte Somma.
Amphibole minerals can be found in all of these mineral forming environments
Tschermakite is found as micro-crystals in cavities in the rock.

Olav Revheim Feb 24th 2011




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Edited 16 time(s). Last edit at 09/27/2013 07:30AM by Olav Revheim.
avatar Re: Tschermakite series
February 17, 2011 08:50AM
Olav,
I originally thought that each mineral should have its own thread and don't know how grouping minerals like these all together will work out but am somewhat guilty of doing that sort of thing myself and the idea is growing on me. We will just see how it is going to work out and how much trouble it will cause when we have to shift all our work into a database format. If we have to, we can always break them out into separate threads although right now it seems sort of silly to do that.

Rock Currier
Crystals not pistols.
Re: Tschermakite series
February 17, 2011 12:53PM
    
Rock,

I see your point, and I think that one entry pr. mineral should be the general rule. In some cases this is not very practical. For Quartz, Beryl and other common, much sought after minerals, it makes sense to split the mineral entry into variety or geographical articles (or possibly both).

I think the amphibole-group with all it's complexity fall in the other end of the scale. They have very complex chemistry and are nearly impossible to distinguish. Some of these minerals are quite rare ( or at least rarely identified). I therefore think it makes sense to treat the series of minerals ( such as tschermakite-series or hornblende-series) as an individual mineral entry also when the articles are transfered into a database format. From a database perspective I do not think it should be too difficult to manage multiple mineral species containing the same root name as one group.

The tschermakite-series is a good example to illustrate why I think treating the various amphibole group mineral species in series articles rather than as individual minerals:

- There are currently 6 approved minerals in this series ( This will probably change with time)
- Only 5 of these minerals have photos uloaded to mindat from 6 different localities.

Of these minerals and locations the status is as follows:
- One is misidentified: kaersutite from Lukov u Bíliny, Czech Republic mislabeled as Tschermakite
- one is almost certainly misidentified (Ferro-aluminotschermakite-Aris,Namibia)
- two entries are questionable ( Aluminotschermakite from Longido, Tanzania and Ferri-Ferrotschermakite Kaveltorp
- leaving only two photo uploads with good references (Ferrotschermakite, Dookie, Australia and Tschermakite San Vito, Italy).

On a side note, it should be noted that multiple amphibole minerals with similar appearance is identified in the San Vito quarry, making the identication of an individual amphibole crystal probably somewhat uncertain.

This leaves only two photos of two tschermakite-series species from two locations reasonably confirmed. I think this is a too weak foundation for 6 "best of" mineral articles.

I am open for discussion on the matter, and it does not take much time to split the articles into individual entries, but I think I am questioning the value of individual article entries for each of the amphibole-group species given their very close relationsship, appearance, rarity and difficulty to distinguish.

smileys with beer

Olav
avatar Re: Tschermakite series
February 18, 2011 02:30AM
In cases like this we try out different things and learn as we go along. Eventually the best way will make itself known.

Rock Currier
Crystals not pistols.
avatar Re: Tschermakite series
February 25, 2011 10:58AM
Olav,
Good job. The get easier and better as you become more familiar with the medium. I went in and did a little adjustment to the formatting. If you would like to see how I adjusted the code strings to tweak the size of the images, click on the edit button at the bottom of the article and examine the code strings. I think the changes will be easy for you to see.

One further thing, you should add these minerals to the fast navigation list of minerals where the first draft is more or less complete. If you run into trouble, rattle my cage.

Rock Currier
Crystals not pistols.
Re: Tschermakite series
February 25, 2011 12:07PM
    
Thanks Rock smiling smiley

I see what you did with the tall image, and must admit that the idea of making it slightly wider never occured to me.:S

I will update the fast navigation and also the amphibole group article

Olav
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