Double Your Pleasure, Double Your Fun
I’m always on the lookout for uncommon pyrite crystals. Iron cross twins have long held a certain fascination for me.
But First, What Is An Iron Cross Twin?
A twin crystal consists of two or more individual crystals joined following a defined crystallographic rule: along a common plane, rotated around a common axis or inverted around a common point. Not every grouping of two or more crystals is a twin. Like not all pairs of siblings are biological twins. To be a twin crystal, the individual crystals must be joined following a defined crystallographic relationship. Similarly, only pairs of siblings having the necessary defined biological relationship are twins. An Iron Cross Twin is a kind of penetration twin that consists of interpenetrated pyritohedra. A classic Iron Cross Twin appears as if one pyritohedron, rotated 90 degrees, is fused at the core of another pyritohedron.
I have several iron cross twins from various localities. Usually they are rather small from sub-millimeter to 1.5 centimetres in size. Pyrite iron cross twins often have a surface alteration to an iron oxide mineral such as goethite. If the altered mineral is not known, it is best to label it as limonite as limonite is a rock of indeterminate composition. These are often labelled as pseudomorphs of goethite or limonite after pyrite. It is not clear to me if the pyrite has completely or only partially altered. There is also some debate over how much alteration is required before it constitutes a pseudomorph. To simplify things, I include all of these in my pyrite collection.
Iron Cross Twins From Different Locations In My Collection
I picked up an iron cross twin from an unnamed location in France at the Nova Scotia Gem & Mineral Show in Parrsboro last year https://www.novascotiagemshow.com/ ( a great show to check out if you get the chance). I usually don’t seek out samples with such general locations (or when no locality is given) but I make exceptions particularly when a specimen exhibits a certain trait. This piece was interesting to me not just because it was from a new locality but also because it was the first time I found a sample at a general gem and mineral show.
The following specimen is from Morocco. I like it because it combines my interest in pyrite and Morocco.
I have several iron cross twins from the Detmold area of Germany. Some on, some off matrix. The pyrite twins are generally small ranging in size form a mm or two to a cm. They typically have some alteration to limonite at least on the surface. Often these are labelled simply as from Extertal, Lemgo or Vlotho without a specific quarry given. I do have a couple labelled as coming from the Maibolte Quarry.
Mindat location references for these sites are:
More recently I have acquired a few from the recent find in Colombia.
While other iron cross twins I have seen are typically small, the Colombian samples are quite a bit larger. One is labelled as from the Matacana Mine, Gachalá Municipality https://www.mindat.org/loc-109643.html. The other two are more generally labelled as from Gachalá Municipality, Guavió-Guatéque mining district, Cundinamarca Department, Colombia https://www.mindat.org/loc-109641.html. These range in size from 2.3 x 2.0 cm to 4.8 x 4.0 cm. I have heard (but not seen) that they can be up to 15 cm!
The Colombian iron cross twins also seem to have at least a surface alternation to limonite (goethite?)
The La Rioja area of Spain is world renowned for its spectacular pyrites, particularly the nearly perfect cubes with mirror-like faces. While cubes seem to be the predominant crystal form, other forms are also found.
I have a bunch of iron cross twins from Canales de la Sierra, La Rioja, Spain https://www.mindat.org/loc-247990.html. The iron cross twins are loose crystals ranging in size from 3 mm to 7 mm across.
Iron cross twins are found a several other places around the world – check out my Mindat article to see where: https://www.mindat.org/article.php/3872/PYRITE+-+Iron+Cross+Twins
While found in several locations, iron cross twins seem to be a rare but well-known morphology. How common are iron cross twins? Check out Part 2 here: https://pyritemania.com/2020/05/27/pyrite-matters-iron-cross-twins-part-2/