Fluorite from the Milpo mine in Peru is instantly recognisable. The octahedral forms, vivid colour and mineral associations are remarkable among fluorites worldwide, and they only become more interesting as you immerse yourself in the locality.

The classic combination is green fluorite with pyrite, and the contrast can be spectacular. Colours range from bright apple green to deeper, more saturated greens, sometimes concentrated within an otherwise highly transparent crystal. The fluorite usually presents as octahedral or modified octahedral, while the pyrite beneath it often occurs as brilliant metallic druse or sharp individual crystals.

Milpo was known to collectors long before the current generation of green fluorites appeared, but finds in 2022 and 2023 revived interest in the locality.

In July and August 2022, a small quantity of green fluorite associated with unusually well crystallised geocronite was recovered.

Specimens arrived on the international scene that year and were shown at Munich, where the combination of apple green fluorite and metallic geocronite was presented as a hot new discovery.

The geocronite was identified from formal analysis, which is an important point to note for when we look deeper into the rather complicated relationship between geocronite and jordanite.

A further find in March 2023 produced pale to medium green fluorite, predominantly as transparent octahedra reaching around 3 cm. Some cubic fluorite was also found. The crystals occurred with drusy pyrite and geocronite-jordanite, including sulphosalt crystals reaching around 7 mm.

Around 35 specimens from the find appeared at the Munich Show later that year, ranging from thumbnails to small cabinet pieces. Contemporary reporting in The Mineralogical Record records prices between €300 and €2,400, with the group selling quickly.

Mineral Associations

Milpo fluorite becomes especially interesting when we look at its mineral associations.

Milpo is primarily a lead and zinc mine, rather than a mine for fluorite. The main minerals being extracted are galena and sphalerite, but they formed beside a wider variety of minerals that include:

• Pyrite

• Sphalerite

• Galena

• Geocronite

• Jordanite

• Boulangerite

• Arsenopyrite

• Bournonite

• Quartz

• Calcite

Pyrite is easily the most abundant association represented alongside fluorite in documented specimens, followed by sphalerite and the geocronite-jordanite series.

What this means for collectors is that you can have several specimens from Milpo, and each can demonstrate an incredibly different expression of the locality.

One specimen might be a clean green octahedron perched on fine pyrite, while another could have a fine peppering of black sphalerite punctuating the crystals. Another might contain galena, or have metallic geocronite-jordanite embedded within the fluorite itself.

It's this incredible range that makes Milpo especially rewarding to collect as a locality, rather than simply as a source of green fluorite.

Geocronite-Jordanite

This is where Milpo becomes wonderfully complex.

Geocronite and jordanite are both uncommon lead sulphosalts, and both have been confirmed from the mine. They're closely related chemically and can be extremely difficult, or impossible, to distinguish reliably by appearance alone.

Their formulas demonstrate the similarities:

Geocronite: Pb₁₄Sb₆S₂₃
Jordanite: Pb₁₄As₆S₂₃

The challenge in distinguishing the difference between them was highlighted when the modern Milpo fluorites entered the market. Different dealers were identifying apparently similar metallic crystals as either geocronite or jordanite. The Mineralogical Record addressed the disagreement directly in its report from the 2023 Munich Show.

“Lacking analysis of a specific piece in question, we should write geocronite-jordanite.”

We agree that this is probably the most useful rule for collectors.

Both types have now been analytically confirmed at Milpo, including identifications using X-ray diffraction and SEM-EDS.

It should be noted that in 2023 The Mineralogical Record considered some of these crystals exceptional for either species (both Geocronite & Jordanite), with the sulphosalt becoming a significant collectible mineral in its own right.

The fluorite happens to be very good company.

Boulangerite

Another interesting association is boulangerite, a lead antimony sulphosalt that can form fine, needle like crystals that can be seen suspended within fluorite.

Boulangerite is considerably less common than pyrite, sphalerite or geocronite-jordanite in documented Milpo specimens. Confirmed and reported examples add yet another intriguing inclusion to an already diverse line up of associatons.

Is Milpo fluorite safe to handle?

The presence of arsenic and antimony in some Milpo associations can sound alarming, but these elements are chemically bound within minerals such as geocronite-jordanite. An intact specimen can be handled and displayed as a mineral specimen with sensible precautions.

As with any specimen containing lead, arsenic or antimony, avoid creating or inhaling dust, don't lick or ingest the mineral, and wash your hands after handling. These are sensible habits more generally when handling sulphides and sulphosalts, including familiar minerals such as pyrite.

There's no reason to be frightened of these specimens when the chemistry is respected. Although, perhaps don't follow my example and eat lunch while filming the latest release.