AN OUNCE OF OSUMILITE (NIFTY MINERAL)

If you believe in science, like I do, you believe that there are certain laws that are always obeyed.             Stephan Hawking

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology.                      Carl Sagan

If the facts don't fit the theory, change the facts.   Unknown

We have come to Day ? of our self-quarantine and what day of the month is it?  Just like the movie Groundhog!  The quotes above just seem to fit my current mindset.  But, I am an optimist at heart and look at the good things in our lives—like sunsets!

The Cascade Volcanoes are the best-known volcanic mountains in the lower 48 states.  The high peaks dominate the Cascade Mountains that extend from British Columbia, Canada, south to California.  The general public is fascinated by the 14,000+ foot Mt. Rainier with its glaciated topography to Mt. Saint Helens that blew its stack in 1980 to Crater Lake with its pristine lake inside the caldera. In southern Oregon is an area well known to hikers and Native Americans as the Three Sisters, each volcanic peak is in excess of 10,000 feet in elevation.  South Sister erupted about 2000 years ago while Middle and North Sisters are fairly dormant (so they say).  Most of the area around the Sisters is protected as a Wilderness (official designation) including a piece called Obsidian Cliffs.  Obsidian is natural glass that forms when rhyolitic magma, the volcanic extrusive of granite, cools very quickly and cannot form crystals.


The volcanic Cascades are the result of a remnant of the Pacific Farallon Plate, the Juan de Fuca Plate, subducting under the North American Plate.  The Farallon Plate, an oceanic plate, was formed along a Pacific Ocean ridge (modern terminology) similar to the current Mid Atlantic Ridge and was transported east (current direction) pushing several “microcontinents”  into the North American Plate and “sticking” them to the Plate. In time, the heavier Farallon Plate was almost completely subducted under the lower density North American Plate leaving behind a few remnants such as the Juan de Fuca.  Another possibility is that the North American Plate overrode several different Pacific trenches (rather than a single Farallon Plate) while incorporating microcontinents into our North American rock column. At any rate, both theories allow for massive mountain building and associated volcanism as either the Farallon Plate or the oceanic trenches were subducted.  The North American Plate movement and oceanic plate subduction created heat and pressure while melting the subducted rocks and releasing energy on the surface in the form of massive volcanism, and mountain building.  Literally the entire western United States has large-scale exposures of extrusive and intrusive volcanic rocks such as basalt, quartz latite and rhyolite (and its relatives welded tuff and ash falls).

Diagrams of Juan de Fuca Plate. Public Domaine but Courtesy of Alattaristarion.

The mountain building and volcanism started in the Jurassic and continues today as noted by the explosion of Mt. Saint Helens in1980.  The Cascade volcanoes are the result of the Juan de Fuca Plate being subducted with resulting deep melting and surface volcanism.

As a soft rocker and paleontologist I never paid much attention to the explosive and extrusive rhyolite, welded tuff, and other siliceous volcanics that are usually a mixture of quartz, sanidine, and plagioclase, until I discovered topaz crystals out in the Thomas Range of western Utah (see Posting     ), and red garnets in the Ruby Range of Nevada, and at Ruby Mountain, Colorado (see Posting May 20, 2014).  There were also some much smaller crystals in the rocks such a biotite, hornblende, and pseudobrookite (see Posting  ) that did not attract much of my attention.  Little did I know that a nifty mineral called osumilite was also lurking in the rhyolite of Obsidian Cliffs, Oregon, and a few other localities. 

Submillimeter (~.75 mm) crystals of osumilite from Obsidian Cliffs, Oregon.

Osumilite grabbed my attention since the mineral is a very rare potassium-sodium-iron-magnesium-aluminum silicate that has very tiny, nicely formed, tabular, hexagonal crystals.  They usually have a blue color that is dark enough to appear black, a shiny adamantine luster, are transparent to translucent, and are somewhat hard at 5-6 (Mohs). The mineral just sort of “grabs you” when looking at nondescript rhyolite under a scope and up pops this nifty crystal tab.  Besides rhyolite, osumilite sometimes appears in volcanic dacite, and in high grade (ultra-high temperature) metamorphic rocks.  Once observed, you will not confuse osumilite [(K,Na)Fe,Mg)₂(Al,Fe)₃₍Si,Al)₁₂O₃₀] with other minerals.  Electronic gizmos can determine which cation is dominant, iron or magnesium, so a mineralogist can add –(Mg) or-(Fe) on the end of the formula!


As we continue to shelter in place I continually think about the future and what it will bring.  And why, why, why?  Is anyone to blame?  But mostly I think about my good life and all that it has brought, and I am well and safe.


Everything happens for a reason' is something that we have to tell ourselves all the time, because it's good to have the idea that something good is around the corner.

        Margot Robbie