ASTROPHYLLITE: A MINERAL FROM THE STARS? NO, ST. PETERS DOME, COLORADO.

ASTROPHYLLITE CRYSTALS FROM SAINT PETERS DOME AREA, PIKES PEAK MASSIF.  WIDTH OF SPECIMEN IS ~2.2 CM.

Astrophyllite is another one of those minerals that would have never popped into my mind if not for a move to Colorado Springs.  Somehow I don’t remember anything about this mineral from my basic mineralogy class; however, that was decades ago and much of that class information seems lost in the deep recesses of my mind!  I certainly never saw the mineral in Kansas, or Missouri, or Wisconsin (my other homes).  But, in exploring around the Pikes Peak Massif near Colorado Springs I heard about this brass- to golden-colored bladed mineral that was found in some of the pegmatites and “granites”.  I guess the name, “star leaf”, comes from the fact that at some localities the mineral occurs as “star bursts or rosettes”; however, all of the Colorado specimens that I have seen are rather bladed or tabular, soft (~3 on Mohs scale), some perfect basal cleavage, and sort of a greasy luster.  It really doesn’t look like much of any mineral that I observed previously except perhaps phlogopite.  However, this mica has cleavage plates that are transparent and flexible.  Astrophillite plates are brittle and opaque.

Astrophyllite is a rather complex mineral, at least to me, a hydrous potassium sodium iron titanium silicate: (K,Na)₃(Fe,Mn)₇Ti₂Si₈O₂₄(O,OH)₇.  I’m just glad that chemical formula was never on a test!

Astrophyllite acquired at Tucson, 2014.  Collected from the Khibiny Massif, Kola peninsula, Russia.  Note the golden-orange chatoyancy of some crystals.  i presume the matrix is something like albite.  Width of specimen ~5cm. 

OK, since astrophyllite did not arrive here from the stars, why is it here near Pikes Peak, one of the few known localities in the world (others seem to be Brevig, Norway; Mont-Saint-Hilaire, Quebec, Canada; Greenland; and the Kola Peninsula, Russia)?  It seems to occur with quartz, feldspar, thorite, galena, riebeckite, and zircon in both pegmatites and fractures of the Mount Rosa Granite (Eckel and others, 1997).  So, I really don’t know the answer to why.  What I do know is that the igneous rocks of the Pikes Batholith around Saint Peters Dome (Precambrian,~1.08 Ga) seem to produce a suite of minerals where many, if not rare, are certainly uncommon. 

GOLDEN BLADES OF ASTROPHYLLITE FROM SAINT PETERS DOME.  WIDTH OF SPECIMEN IS ~2.5 CM.

REFERENCES CITED

Eckel, E. B. et al, 1997, Minerals of Colorado: Denver Museum of nature and Science and Fulcrum Publishing, Denver.