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Identity HelpGuatemalan Serpentine? If so, what kind?

28th Nov 2016 03:21 UTCRobert Darabos

Today I found a large outcrop of this material in a cliff side in Huehuetenango, Guatemala.

I initially thought of Serpentine.

It has a waxy-like feel to it, scratches easily with a steel knife, and has no reaction to vinegar.

A close up shot shows some black and brown dots on the matrix, which I thought to be Manganese and Iron oxides? There are also small crystals, which I assumed were Quartz or Calcite?

Can anyone confirm these ideas?

And any ideas what kind of Serpentine it is, if that is what it is?

Thanks for the help

28th Nov 2016 04:29 UTCGregg Little 🌟

Serpentine is serpentine as there are not really any "kinds". Serpentine can occur with many other minerals as your sample shows. There may be a qualifier as to what kind of serpentine bearing rock it is but as calcite is calcite and quartz is quartz, serpentine is serpentine.

28th Nov 2016 06:23 UTCAlfredo Petrov Manager

:-S http://www.mindat.org/min-11135.html

28th Nov 2016 16:20 UTCRobert Darabos

There are several different kinds of Serpentine. And Quartz. And Calcite.

28th Nov 2016 16:39 UTCReiner Mielke Expert

Could be a talc schist. There are different varieties of quartz and calcite but not kinds, since kinds implies species.

29th Nov 2016 01:30 UTCRobert Darabos

What about green Diorite? Just was reading about that.

29th Nov 2016 01:34 UTCReiner Mielke Expert

You cannot scratch diorite with a knife.

29th Nov 2016 03:32 UTCStuart Herring

Hi Robert... the first thing to do is see if any bed rock geological maps exist for that area. Second test it with a magnet (I see dark areas in the pics). If it is "magnetic" then it has a iron mineral - most likely magnetite. Third - can you scratch it with your thumb nail ? - if so you have talc.

The specimen looks like talc... but the low contrast of the pics -ie color makes it hard to tell. Second guess serpentine... which is series of minerals... NOT just serpentine.

29th Nov 2016 03:36 UTCPaul Brandes 🌟 Manager

Soft, waxy feel and no reaction to vinegar....... sounds like something mixed with talc or maybe a talc schist as Reiner suggested.

29th Nov 2016 05:32 UTCGregg Little 🌟

Sorry Alfredo, I should have been more specific in saying that serpentine is a group of minerals and in rock specimens like the one pictured they present much the same characteristics. In the case of this rock sample, as there seems to be talc present, then the label might read "Serpentine altering to Talc". In the alteration sequence (to lower temperatures and pressures) basic and ultramafic igneous rocks alter to serpentine then to talc.

Serpentine alterations is usually associated with more basic to ultramafic parent rocks (decreasing quartz and increasing mafic mineralogy) which diorite, although intermediate in composition, can occur within areas of basic igneous rocks. As a rule of thumb diorites are a somewhat darker grey colour unless they are quartz diorites but then we are now grading from intermediate to acidic igneous rocks (ie. granites).

1st Dec 2016 01:48 UTCRobert Darabos

I for some reason did not even think to try and scratch it with my fingernail ! Thanks for the reminder.

It does not scratch with fingernail, so that takes out talc, right?

1st Dec 2016 12:08 UTCReiner Mielke Expert

What does not scratch with the fingernail? The greenish stuff with the foliation or the granular white stuff?

1st Dec 2016 17:01 UTCD. Peck

Robert

The reverse is true . . . that which you can scratch with your fingernail is probably talc.

1st Dec 2016 19:58 UTCRobert Darabos

Nothing on the specimen scratches with a fingernail. The white crystals or the green 'stuff'

2nd Dec 2016 16:53 UTCD. Peck

Hi Robert, I have got to learn to read. :-(

2nd Dec 2016 18:01 UTCGregg Little 🌟

Don, I don't think there is much to apologize for as the earlier descriptions of the rock do lead to confusion as this is a rock that appears to have been subjected to much retrograde metamorphism; alteration to a mixture of lower temperature and pressure minerals.

Roberts earlier description of a waxy feel would lead one to assume talc and there was the additional information of softer than steel but harder than finger nail. This rock could be a mixture of quartz, serpentine and talc which gives a "blended hardness?" of 1 to 7 arriving at what "apparent hardness". To further illustrate, scericite alteration is a mixture of muscovite and quartz so what would that composition offer in hardness determination?

This is the problem with asking to identify rocks which are mostly usually a mixture of minerals and frequently finer grained making it tough to identify by any pictures given and only generalized physical properties. In these cases seeking out local sources, be it literature or people, is much more conclusive.

2nd Dec 2016 21:23 UTCD. Peck

Maybe we should ask for photos of a thin section for rocks! :-D

2nd Dec 2016 23:37 UTCGregg Little 🌟

Wouldn't that put us in the high-priced (paid) category and what is the magnification on that microscope your sitting in front of?

3rd Dec 2016 03:27 UTCD. Peck

Hi Greg, I hope everyone realized that I was joking! I have cut my own thin sections, but I don't recommend it. The scope in front of me in the photo is a really old Bosch & Lomb. It has a horizontal rotating barrel turret for the objectives (3) that are 0.7x, 1.5x, and 3x. I had 10x and 15x oculars (almost never used the 10x). I don't know the age, but I would guess it came from the 1930s or so. I no longer own it. The objective lenses were cemented with Canada balsam which went bad. It was mounted on a 40 pound pillow block, which I liked. i could swing the scope out of the way when I needed the space for working. I used it for about 30 years with a lot of satisfaction, but it can't match the Meiji that I use now.

3rd Dec 2016 06:15 UTCGregg Little 🌟

Yup Don, I got it and most should have with the emoticon at the end.;-) I too cut thin sections for a dissertation on volcanoclastics but that was what seems like eons ago. Couldn't be bothered now, would rather stick to my Nikon binocular microscope, vintage 1974 with great optics. Started mineral collecting (micromounts) with it and still use the scope in my field work for oil and gas exploration.

24th Dec 2016 17:08 UTCRobert Darabos

Could this material possibly be Lawsonite? I found these two photos of low-grade Lawsonite and it looks very similar in color and appearance. The hardness is also very close to the same, just above Quartz. Lawsonite is also found in several regions in Guatemala...

25th Dec 2016 00:24 UTCRalph S Bottrill 🌟 Manager

The original photos look to be a sericitised quartz porphyry, or quartz-phyric rhyolitic tuff. You really need a thin section for these rocks.

25th Dec 2016 07:30 UTCVolkmar Stingl

For me it looks like a sericite quartzite, not a rhyolite.

26th Dec 2016 20:54 UTCRalph S Bottrill 🌟 Manager

It doesnt look quartz rich enough for a quartzite, but could be a highly sericitised arkose or similar rock; again hard with a thin section or knowing the local geology.

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