Hedenbergite versus Johannsenite

Mineral Matters #438

In the past, a lot of specimens in the hedenbergite-johannsenite series from various mines along the Broken Hill line of lode were labelled as Fe-johannsenite. However, at a point in time, it was determined that for it to be johannsenite, it needed to not just have Fe greater than Mn, but (Fe+Mg) greater than Mn.

As a result, the vast majority were designated as Mn-rich hedenbergite. Also as a result, the only true johannsenite specimens came from the North Mine. But crucially, not all North Mine specimens are johannsenite!

So unless your specimen has been analysed, you should probably call it Hedenbergite-Johannsenite Series. It is likely though, based on the many analyses that have been performed over the years, that hedenbergite is probably a safe bet, particularly if from mines other than the North Mine.

The first specimen below is probably Mn-rich hedenbergite. It consists of green crystals with pink to brown bustamite. This specimen came from a mineralogy professor, labelled as johannsenite, and without a specific mine name, and I doubt that it was analysed.

The second specimen is also labelled as johannsenite, from the North Mine, and occurring on galena. This may have been analysed (but I am not certain). However, the association with galena fits the reports.

Below: Hedenbergite (likely), Bustamite, Broken Hill, New South Wales. Width of view 6mm. Click on the image for a higher resolution version.

Below: Johannsenite (possibly), spessartine and galena, North Mine, Broken Hill, Broken Hill district, Yancowinna Co., New South Wales. 21mm wide.