AEGIRINE: INTERESTING MINERAL

AEGIRINE AND FELDSPAR.  WIDTH ~4 CM.

 At a small rock and mineral show a few days ago I saw a specimen that sort of intrigued me since the presence of numerous large crystals sort of reminded me of epidote, or something close.  However, the label stated “aegrine” that I took to mean aegerine or aegirine.  It was an older label (Albuquerque Gem and Mineral Club) and the seller informed me that it was purchased as part of a previous collection.  At any rate, it was not very expensive and I was interested.

After returning home I rattled my mind to recall something about aegirine (preferred spelling of MinDat.org) but that was not too successful except to remember that it may be a pyroxene.  After that knowledge, “things” got complicated for a non-mineralogist!  

I know that the pyroxenes have a general formula of (Na,Ca)(Mg,Fe,Al,Ti),(Si,Al)₂O₆, or something close.  In the clinopyroxene subgroup diopside is the calcium/magnesium-rich end member and at the other end of the solid solution series is the calcium/iron-rich member, hedenbergite.  Somewhere in the middle is the very common rock forming mineral, augite.  In addition, jadeite is a sodium/aluminum end member.  What I needed to better understand was the relationship of aegirine to augite and other pyroxenes.  So, I decided to contact  Pete, my “go-to answer man” up at the USGS.  What I found out is that in aegirine (NaFe(+3)SiO₂O₆) there is a coupled substitution where Na replaces the calcite and Fe(+3) replaces the magnesium.  And, the possibility exists that aegirine may be in solid solution transition with any of the other  members with the most common transition being aegirine-augite (and that is what the hybrid is called).  Those tidbits are about all that I pretend to understand, or want to understand  when Pete tells me the mineralogy is somewhat more complicated!

AEGIRINE CRYSTALS.  WIDTH PHOTO ~3.5 CM.

At any rate, I have a nice little specimen with prismatic crystals (Monoclinic) of vitreous aegirine, black in color (may be sort of a greenish-black), and a hardness of 6 (Mohs).  Some of the crystals are terminated and they come in all sizes.  They are associated with large feldspar crystals, orthoclase, and very tiny crystals of zircon and ?quartz.  The specimen comes from Mount Malosa, Zomba District, Malawi, where outcrops of “alkali pegmatites [contain] unusual REE minerals [and are] famous for excellent and large aegirine crystals, feldspars, arfvedsonites and rare Be minerals” (MinDat.org).

I love to learn and specimens like this, along with a “go-to answer man”, make life quite enjoyable.

PHOTOMICROGRAPH OF ZIRCON CRYSTAL ~54 MM.

PHOTOMICROGRAPH OF ZIRCON.  TOP CRYSTAL ~.51 MM.

PHOTOMICROGRAPH OF CRYSTAL ~1.2 MM.  INITIALLY I THOUGHT INCLUDED QUARTZ BUT NOW UNCERTAIN.