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Improving Mindat.orgtype photo of howieite: is this deerite?

4th Sep 2012 15:17 UTCTony Peterson Expert

I recently obtained a Longvale specimen, containing lilac-grey crystals which I had analyzed by EDS and conformed well to howieite, including the presence of some Ca, Ti-rich grains (see http://www.mindat.org/photo-484865.html).

The type photo, however, shows distinctly brown crystals, which is a much better match to deerite (compare http://rruff.geo.arizona.edu/doclib/hom/deerite.pdf to http://rruff.geo.arizona.edu/doclib/hom/howieite.pdf). The specimen label included as a child photo states it's a deerite specimen, "with" howieite (http://www.mindat.org/photo-311338.html).

Is it possible the contributor photographed the wrong species on the specimen?

Tony Peterson

4th Sep 2012 20:17 UTCKeith A. Peregrine

Tony,

Do you have a photo of the lilac-grey crystals? I'm quite curious about them. The possibility of Howieite being this color is most striking. Everything I've read and seen is that it is black. I've not seen brown or greenish color Howieite, though I admit much of my material from Laytonville has not been tested. Once back home, I'll dig out my copy of the Guide to Mineralogy of the Laytonville Quarry by Charles Chesterman. A bit old, but the mineral discussion is still relevant today.

Keith

4th Sep 2012 20:31 UTCAlfredo Petrov Manager

A few other factors that might not be important, but might be worth considering:
Size of crystals - Lots of "black" minerals show other colours when they get small enough to let light be transmitted, and those crystals on that excellent photograph you link to are much tinier than I've seen in other photos. Even "black" schorl shows various colours when the particle size gets small enough.
Light source - Now that incandescent tungsten bulbs are going out of fashion and those horrid energy-saving interrupted-spectra bulbs are becoming de rigeur, minerals are changing colours! :-X
Pleochroism - Howieite is strongly pleochroic, so orientation of the view might make a noticeable difference in the transmitted light colour.

4th Sep 2012 21:02 UTCTony Peterson Expert

in response....

Keith: my photos are uploaded under Longvale, in two groups, macros under almandine-howieite, micro under howieite-almandine.

Alfredo is of course correct: nearly anything will transmit light if it's thin enough, and neither howieite nor deerite is absorptive enough to prevent that. I will quibble with him on one point: the (incandescent) light I photograph with is not polarized, and will therefore not normally reveal pleochroism. I suspect my howieites look lilac-grey because that is the strongest-absorbing pleochroic color of the mineral (other two are golden and green). ALTHOUGH I am of the opinion that CCD detectors in SLRs have an anisotropic response and can reveal pleochroism; this is why, for example, nearly all digital photos of dioptase, which is a viciously anisotropic and pleochroic mineral, usually come out too blue. Howieite is strongly birefringent so perhaps its pleochroism is being detected here.

Never, never, ever, photograph under LED light, even fluorescent is bad. Continous-spectrum incandescent is the only source to use.(tu)

Oh and Alfredo, thanks for the compliment, but I hope to do better with the howieite photos, I need to get an extension for my lens and should be able to double the magnification, which will be a big help. I have a feeling this Longvale specimen I acquired is exceptional for having free-growing howieite.

Tony

6th Sep 2012 04:19 UTCKeith A. Peregrine

Tony,

Chesterman states that Deerite, Howieite, and Zussmanite are found in Metasedimentary rocks (Metachert, Metashale, Metaironstone, and Metalimestone). His comments on Howieite are as follows: "Howieite is one of the more spectacular of the new minerals found at the Laytonville Quarry. It occurs in dark-green to black-bladed crystals that commonly form plumrose and interlaminated aggregates. Crystals of Howieite up to 1 cm in length are common. From a first glance at a specimen, one might easily mistake black Stilpnomelane for Howieite. However, a hand-lens examination will quickly reveal the prismatic character of the Howieite as compared to the tabular nature of the Stilpnomelane."

The Howieite I've seen have been the plumrose, that is radiating crystal form. All, distinctly black and lustrous under sunlight and artificial lighting.

The blue, elongated crystals in the photo uploaded above with the garnets are found in the blueschist in small vugs most of the time covered with calcite. Chesterman describes Glaucophane as: "Glaucophane is in small interlocking prismatic crystals that range in size from 0.2 to 1.5 mm." Also, Glaucophane gives the blue color to the metabasalt, the blueschist.

The garnets are found throughout the Metabasalt and Metasediment. Chesterman mentions that the garnets "appears to contain both Almandine and Spessartite."

The sample above reminds me of crystals found in blueschist. If you compare your images to those in the Mindat Glaucophane collection, you will see the similarities of the blue crystals. So, the analysis comes to me as quite a surprise. Time to have a few of my samples analyzed too.

As for lilac colored minerals, Lawsonite comes to mind. Chesterman places Lawsonite with the blueschist, but makes no mention of Lawsonite with the metasediment.

Keith

6th Sep 2012 08:30 UTCErik Vercammen Expert

The list of minerals for the locality contains also riebeckite and "crossite".

6th Sep 2012 13:49 UTCTony Peterson Expert

I based my ID primarily on the analysis. In particular, the presence of Ca and Ti-bearing variants matches the EDS results very well. I agree that the mineral I posted resembles ferroglaucophane; I will try to get an XRD identification. It is possible that small inclusions of titanite are responsible for the variation.

It definitely is not lawsonite.

But I do maintain that the brown mineral currently used as the type photo for howieite is more likely to be deerite. Its color is not a match for howieite, and the specimen is labeled as dominantly containing deerite.

Tony

8th Nov 2012 16:05 UTCTony Peterson Expert

I have finally had the mineral on my specimen analyzed by XRD - it is indeed glaucophane. I have edited all of the photos of it. My apologies for the original misidentification as howieite.

I still pose my question regarding the type photo for howieite, I suspect it is deerite.

Tony

11th Nov 2012 13:35 UTCReiner Mielke Expert

Hello Tony,

You also need to change the titles of the EDS spectra to glauocophane. Nice photos and thanks for also posting the EDS, any chance you could post the XRD results as well? Hope you get some response to your Deerite question soon.

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