GeneralViitaniemi albite vs. montebrasite
Hi all,
I've been recently going through many of my Viitaniemi samples again. I started to wonder if all of the material I had assumed to be montebrasite really was it. One should never assume...
As many Viitaniemi afficionados of Viitaniemi know, albite can take many forms at the quarry: small-crystalline "sugar albite", more leafy cleavelandite and various forms of more blocky albite.
A quick rule for finding interesting phospates etc (including väyrynenite) at Viitaniemi says: "Find montebrasite, then you have better chances of finding the phosphates". Many people might assume that the rare phosphates are associated with montebrasite and not albite.
Well, there are many problems with this rule:
- rare phospates are also often closely associated with albite
- albite and montebrasite both have perfect cleavage
- both minerals at Viitaniemi can be white-white or creamy-white
- both minerals can have a duller lustre in blocky form
- both minerals have roughly the same hardness
- both minerals can have the same fluorescence colours or lack them
I strongly suspect that many rockhounds out there may have some albite among their montebrasite-labelled samples. Sometimes, in some cases it is very difficult to separate the two minerals even with a 10x loupe. And the minerals can almost form or mimic stealthy mixtures, it seems.
Polishing small pieces and taking refractive indices will tell which is which, as of course various analytical methods.
I include a few pictures here with some quick-and-rough full-spectrum Raman data, with the characteristic silicate- and OH-peaks. If required, I can add more pictures later on, with more montebrasite cleavage surface views.
A view of albite with morinite:
And montebrasite without any phosphates:
A quick Raman comparison (long spectrum version):
I welcome criticism and comments.
Cheers,
Montebrasite and albite are easy to distinguish the old fashion way. Montebrasite fuses easily at 2, whereas albite does not (at 4). Also montebrasite produces a yellowish red flame whereas albite a bright yellow one. Furthermore powdered montebrasite dissolves easily in sulphuric acid and albite does not. 
Hi Joel,
all this is good of course, but why you don't want to try simple wet chemical test for phosphate ion with molybdate ammonium solution (with lemon-yellow phosphate-molybdate precepitation)!? This test is very simple, express and 100% unambiguous (in absence of U in testing probe, which will produce masking brown coloration).
Sample grain put on watch glass in drop of acid (I like to use 1:1 HNO3), minute later you add to the drop of solution on glass a drop of (NH4)2MoO4 water solution. If the test is positive, lemon yellow coloration appears immediately.
Such way you may to check whole your collection during a couple days. 
Hi Reiner, yes, the fuse test works, I've tried.
Sulphuric acid I have, but don't like using it, nasty stuff if you get even the slightest drop on your clothes or anything else ;-).
Refractive index testing easy for me and non-hazardous.
Pavel, I live in a village of about 3200 people.
The last time I even dared to ask about Canada balsam, some common oils (for optical mineralogy/ microscopy) and so on, they thought I was crazy. And they didn't even know some of the stuff, believe it or not.
I wonder if I can get such analytical chemicals without an official college or research lab connection - and for any reasonable price..? I have no idea, but am interested if possible.
Cheers, 
I always am able to identify albite by its inherent polysynthetic twinning lamellae, very obvious on crystals but also visible even when cleaved (in certain directions) because they fully penetrate the crystal. It may be microscopic, but it is always there in pegmatitic material. Montebrasite does not have it, nor does K-spar. I posted these photos of a very megascopic example to illustrate the point.
https://www.mindat.org/photo-766430.html
https://www.mindat.org/photo-766431.html Harold, thanks, a very educative picture. A good old microscope indeed will usually sort out the problem re albite twinning.
My point actually was about not-so-experienced hobbyists who rarely read up on mineralogy. There is a huge amount of these people who visit or have visited Viitaniemi who lack any techical knowledge.
The problem with Viitaniemi is also that montebrasite can be layered, changing crystallisation direction: it it interesting and confusing and it may resemble albite ;-). Familiar to all Viitaniemi fans, here?
Really nice to get so much feedback and ideas, more than I thought :-) Pavel, excellent. I could hazard that and see if the stuff gets through the customs with no issues.
Thanks! 
Joel.
As Reiner mentioned the flame test works well with a simple small propane unit like the ones plumbers use to solder pipes. I purchased a specimen many years ago labeled as microcline with pollucite and elbaite from a Connecticut lithium-enriched granite pegmatite. It was heavy for feldspar so I took a small chip which fused and gave a red colored flame. I have used this method multiple times.
