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Posted by Olav Revheim  
Olav Revheim October 24, 2010 08:29PM
Click here to view Best Minerals S and here for Best Minerals A to Z and here for Fast Navigation of completed Best Minerals articles.

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? After each set of pictures there should be some descriptive text. If none appears it means that we need someone to tell us about the specimens from that locality and something about the geology of the occurrence.



Sekaninaite 8cm long crystal
Sekaninaite 8cm long crystal
Sekaninaite 8cm long crystal

Sekaninaite is the Fe dominant member of the continuous series between the Mg dominant cordierite and sekaninaite. The physical an optical appearance of sekaninaite is similar to those of cordierite, and before 1975 no distinction was made between the species. Sekaninaite is quite a rare mineral. In the Mindat database only 18 ( as pr sept 2010) locations are registered worldwide. This is rather strange as the cordierite end member of the series is an abundant, rock forming mineral. There are good reasons for sekaninaite being rarer:

- The cordierite-series minerals are predominantly found in magnesium rich environments poor in iron. In environments with more iron than magnesium present, garnets and micas tend to form rather than sekaninaite. This is particularly true in regional metamorphic conditions where the P/T (pressure/temperature) conditions are stable for an extended period of time. Sekaninaite is consequently found in pegmatites and contact metamorphic rocks.

- Also, sekaninaite was not described as a separate species until 1975, any earlier finds was classified as cordierite or Fe-cordierite, so some of the cordierites described in the literature may well be sekaninaites. A typical example is the 1950 article “CORDIERITE IN PEGMATITE NEAR MICANITE, COLORADO” E. Wm. Heinrich, University of Michigan,Ann Arbor, Michigan, indicating that the cordierite contains 60% Fe2Al4Si5O18 and 1,5% alkalies. If this is correct, the “cordierite” is in fact sekaninaite.

Czech republic,
Moravia (Mähren; Maehren), Vysočina Region, Žďár nad Sázavou, Dolní Bory

Sekaninaite 7x7 cm crystal cross section
Sekaninaite 7x7 cm crystal cross section
Sekaninaite 7x7 cm crystal cross section

The best loction for sekaninaite is by far the type locality Dolní Bory in the Bory pegmatite district.
where giant crystals can be found. Sekaninaite occurs as poorly developed crystals up to 60-70 cm
in size, usually twinned on {l l0} and {310}, simulating hexagonal symmetry. They are often partly or fully altered to chlorite and/or mica type minerals.

This pegmatite district is located within the area of granulitic rocks of the Bory granulit massif surrounded by cordierite migmatites and biotite-sillimanite migmatitic gneisses. There are three types of pegmatites in this area:

(i)Symmetrically zoned barren pegmatite dikes with schorl, muscovite, apatite and locally also with andalusite and sekaninaite
(ii) phosphate-bearing pegmatites characterized by the presence of primary Fe,Mn-phosphates
(iii) The most evolved lepidolite-subtype pegmatites with abundant elbaite, locally amblygonite-montebrazite, cookeite and spodumene+quartz pseudomorphs after petalite,

All these pegmatites, whose geochemical characteristics indicate the LCT affiliation, belong to a single pegmatite population likely derived from related parental granites, and are about 350 mill years old.

Sekaninaite are found in an intermediate, albite-rich zone between the outer graphic granite zona and the central zone with blocky microcline and quartz. Sekaninaite occurs with schorl, muscovite, andalusite, biotite and apatite in minor to accessory amounts.

Click here to view Best Minerals S and here for Best Minerals A to Z and here for Fast Navigation of completed Best Minerals articles.

Edited 4 time(s). Last edit at 07/17/2015 01:57PM by Olav Revheim.
Rock Currier October 25, 2010 04:38AM
Good work. I installed the navigation links at the top and bottom of the article and added the mineral to the fast navigation list at the top of the Best Mineral forum.

Rock Currier
Crystals not pistols.
Sorry, only registered users may post in this forum.

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