POLYMORPHS: ANATASE AND BROOKITE

Last Saturday I took the opportunity to attend a local estate auction where the handbill advertised thousands of rock, mineral and fossil specimens.  I thought---sure, “thousands”, no way and they are probably junk.  What a pleasant surprise was in store as there were “thousands” of specimens and while some were junque, many more were very nice and collectable.  Most of the fossils were local Cretaceous baculites and really not collectable.  There were also flats of pretty unspectacular microcrystalline quartz available and some went for quite a few bucks!  Why?  But then there were nice collectable individual specimens thrown into flats (beer cases) with about two thirds carrying correct labels.  Some of the flats contained specimens from a single locality such as unlabeled topaz-bearing rhyolite from western Utah.  Most other flats were mixed with perhaps the most interesting being opal (looked Australian) with barite (almost certainly Hartsel, CO).  I guess the packer saw that both were “shiny”. 

I had a great time visiting with, and bidding against (although very hard), about 9-10 other members of CSMS.  I would guess that the ten of us purchased the great majority of the collectable specimens.  The auctioneer would start out with a flat and request bids for any single specimen or two and then sell the remainder of the flat at a single price.  I thought that some of my buys were “quite good”!

One partial flat that I purchased had a small crystal of anatase, something that was not in my collection and a mineral not all that familiar to me ---but I wanted it.  So for $5 I was able to “get it” along with a beautiful, water-clear, double terminated, scepter quartz crystal, a piece of rhodochrosite (with crystals), a nice terminated apatite crystal, some gemmy-green, titanite (sphene) crystals, and several other specimens.  A great buy.

Driving home I kept probing the back recesses of my mind—what do I know about anatase?  Where had I seen specimen(s)?  Then something popped out, or turned on, and I remembered by blog about the mineral brookite (March 7, 2012).  There must be some relationship!  So, in examining the posting I noted my words, and there it was—anatase:  Brookite is a titanium dioxide, TiO_2, crystallizing in the orthorhombic system.  MinDat notes that brookite is one of five titanium dioxide minerals (rutile, anatase, akaogiite, unnamed) that occur in nature---all belong to different crystal systems!  So, anatase is a polymorph of brookite!

Anatase is in the Tetragonal System while brookite is orthorhombic; both are usually found in primary sources as single crystals.  I say primary since all of the polymorphs are not rare in concentrations of heavy minerals from sedimentary rocks.  Anatase has a hardness of about 5.5-6, an adamantine to metallic luster, and a dark steel blue to black color.  The source for the larger crystals seems to be secondary and derived, via hydrothermal solutions, from titanium-bearing minerals in igneous and metamorphic rocks (www.MinDat.org)

 

The specimen now in my collection has a label indicating that Minas Gerais, Brazil was the place of origin.  All-in-all, it was an exciting day.  Now, off to finding shelf space!

 

Anatase crystal from Brazil partially covered with a “clay mineral”.  Width at junction of crystal and clay is ~5 mm.

Brookite crystal from Arkansas.  Length is ~1.75 cm.

ADDENDUM, 5 June, 2103.  Partial large crystal of anatase from Cuiaba District, Minas Gerais, Brazil.  Length ~2.0 cm.