Tuesday, June 5, 2012



For decades I explained to students and laypersons that the common rock termed “granite” had an easy mineral identification:  about 20% or more quartz, 60% to 80% feldspar (both alkali types such as orthoclase, and plagioclase), and a dark mineral such as biotite and/or hornblende.  There are numerous varieties of granite but all contain either hornblende or biotite.  Wrong!  I just needed to move to Colorado and discover the Precambrian Pikes Peak batholith.  In addition to the biotite-hornblende myth, I thought that the Pikes Peak Granite was just a “big ole pluton” with a fairly homogeneous mineral composition and generally pink in color; the pegmatites present contained larger crystals.  Wrong again!

Smith and others (1999) pointed out that the Pikes Peak Batholith is a composite system (~1.08 Ga) composed of at least two different granite types.  The first is a “potassic” series (~64%-78% by weight of SiO2 with biotite and or hornblende), mainly the “real” Pikes Peak Granite, and a “sodic” series (44%-78% SiO2).  Rocks of the latter series were emplaced via at least seven different smaller plutons, among them the Mount Rosa Granite.  Both of the “series” were emplaced close together in time and space although the Mount Rosa Granite has been intruded into the Pikes Peak Granite (Gross and Heinrich, 1965).  

Near St. Peter’s Dome on the flanks of the Pikes Peak Massif, the Mount Rosa Granite crops out, and its major minerals include microcline feldspar, quartz and riebeckite.  So here, the major dark-colored silicate mineral in the granite is the somewhat rare amphibole, riebeckite (Gross and Heinrich, 1965).  Although most of the Mount Rosa Granite is fine grained, some pegmatites are present (see specimen photo).

Riebeckite is an iron-sodium silicate [Na2][Fe2+3Fe3+2]Si8O2(OH)2 that is unique in that both ferrous and ferric iron are present.  The mineral, with a hardness of 5-6, is usually dark blue to black in color and the crystals are columnar aggregates.  The ends of the individual columns are usually broken and rarely terminated.  My specimen is actually pegmatitic in nature. 

One interesting aspect of the riebeckite-bearing Mount Rosa Granite is the level of radioactivity present, mostly due to the presence of thorium.  Exploration pits have been constructed and 500 tons of ore were processed for their radioactive content; however, commercial processing seems unfeasible (I think) (Gross and Heinrich, 1966).

And finally, a variety of riebeckite termed crocidolite is an “asbestos” mineral with a fibrous habit and dangerous if ingested.  

Gross, E. B. and E. W. Heinrich, 1965, Petrology and Mineralogy of the Mount Rosa Area, El Paso and Teller Counties, Colorado: I The Granites:  The American Mineralogist, v. 50.
Gross, E. B. and E. W. Heinrich, 1966, Petrology and Mineralogy of the Mount Rosa Area, El Paso and Teller Counties, Colorado: III Lamprophyres and Mineral Deposits:  The American Mineralogist, v. 51.
Smith, D. R. and J. Noblett, R. A. Wobus, D. Unruh, K. R. Chamberlain, 1999, A review of the Pikes peak Batholith, Front Range, Central Colorado: A “Type Example” of A-type Granitic Magmatism: Rocky Mountain geology, v. 34, no. 2.