A NICE BLUE BORON MINERAL: DUMORTIERITE

At one of the rock shows in Arizona this spring I saw a beautiful cabochon labeled “Blue Quartz” with attributes of being able to stimulate new and better relationships, and to remove personal fear of others.  Although I am far from a mineralogist, something didn’t look quite right to me--quartz it was not. The gentleman selling the cab did volunteer that that the original pre-cab rock came from an old collection that in turn was garnered from “a clip mine near Yuma”.  He was uncertain what a clip mine was!

 

Top section of dumortierite slab.  The pyrophyllite? seems stained with iron.  The upper portion is “fresh” dumortierite from a “break”.  The slab is ~ 12 cm. in length.

I inquired about “blue quartz’ later on in the month at a dusty little rock shop.  Wow, the proprietor took me over to a beer flat and showed me slabs of blue material and told me it was a local mineral called dumortierite---and came from the Clip Mine down near Yuma in southern Azizona.  Serendipity!  So, I purchased a nice slab of the mineral complete with some wonderful “blue markings”.

 

Reverse of above photo; slab is sawed.  Note that I am uncertain if both shades of blue, light and dark, represent dumortierite, or if they are two different minerals.

Dumortierite is a a rather uncommon aluminum borosilicate, (Al.Fe)₇BO₃(SiO₄)₃O₃.  Although it may form nice slender crystals, most of the time the mineral is composed of  fibrous aggregates that generally are blue to violet in color (although green, pink, brown and green colors sometimes appear).  Dumortierite is about as hard as quartz (7 Mohs), has a vitreous luster, and occurs both in metamorphic and igneous rocks (aluminum and boron rich).  The rock shop proprietor told me that polished specimens/slabs are commonly observed in southern Arizona and adjacent California but seem rare elsewhere.

Photomicrograph of slab—note reflected parallel saw mark.  White mineral is quartz. Width of blue band is ~ 1 cm.  

The specimen that I purchased is a mixture of dumortierite, quartz, a soft clay-like mineral (probably pyrophyllite) and some other unidentified minerals. In places the pyrophyllite? on the slab seems covered by iron staining.

As for the Clip Mine, www.mindat.com noted it is “a former underground Pb-Ag-V-Fe-Mn-Sr-Baryte-Fluorspar-Cu (Cl-Br) mine…located about 28 miles north of Yuma, Arizona… Mineralization is a linear ore body with argentiferous, oxidized lead minerals with silver chloride and bromide, vanadinite, and some malachite in a gangue of ferruginous and manganiferous calcite, quartz, fluorite, barite, pyrolusite, iron oxides, celestine (?), gouge and brecciated wall rock, in a lensing vein in a fault zone cutting Tertiary andesitic to dacitic flows, tuffs and breccias…It was mined mainly in 1883 to 1887 and to a lesser degree in 1925 through 1929… Ag/T. Silver produced was more than $1,000,000 (period values) and an additional 7,000 oz. Ag (1928-1929).”

I am going to leave my specimen of dumortierite right here on my desk since it has a strong ability to aid intellectual activity within the brain.... and will assist [me]to develop enhanced mental abilities (Healing Crystals for You, 2013).  I certainly need all the help that I can get.