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Mineralpedia Details for Jahnsite-(NaFeMg)




Jahnsite - (NaFeMg) occurs in at the Tip Top pegmatite west of Custer, South Dakota. Thus far it has been a one-time find. Crystals occur as 1mm pseudo-orthorhombic twinned prisms to 1mm in tiny vugs within Heterosite with dark purple Leucophosphite and Dufrenite. The crystals are translucent yellow with orange bands near the termination. Two other new Jahnsites (NaMnMg) and (CaMgMg) are present in the crystals but can be characterized due to insufficient volumes.

Ref. Kampf, A.R., Steele, I. M., Loomis, T. A. Am.Min. 93,940-945(2008).

Crystal System
Crystal Habit
Prismatic, Striated 
Vitreous (Glassy) 
Monoclinic - Prismatic 
Brittle - Splintery 
View Jahnsite-(NaFeMg) 
View Jahnsite-(NaFeMg) 

Jahnsite-(NaFeMg) from Tip Top mine, Custer Co., South Dakota, United States

            from Tip Top mine, Custer Co., South Dakota, United States
Special Info
Type and Only locality

Tiny 0.1 to 1mm, twinned, prismatic yellow crystals with orange terminations.  Anthony Kampf described these crystals as having five different chemical zones.The center portion of the crystal, which is the central part of the yellow zone just below the orange band is Jahnsite (NaFeMg). The base, which isn't shown in the photo is the yet to be described Jahnsite (NaMnMg). The orange band I believe to be the original Jahnsite (CaMnMg) and the above the band is the again Jahnsite (NaFeMg) which finally phases into another new, not yet described Jahnsite (CaMgMg) at the tip. According to Kampf et al (2008), 80% of the crystal by volume is Jahnsite (NaFeMg). There were insufficient amounts of crystal to perform the necessary work to fully describe the other two new Jahnsites.

Quotes from original text Kampf, A.R., Steele, I. M., Loomis, T. A. Am. Min. 93, 940-945(2008).:

"Typical crystals have as many as five chemical zones from base to termination corresponding to the following jahnsite species: jahnsite-(NaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMgMg). The characterization of the new mineral jahnsite-(NaFeMg) is based upon the second zone listed, which corresponds to roughly 80% of the crystal volume. Jahnsite-(NaMnMg) and jahnsite-(CaMgMg) also represent new species...." ...."The color banding observed is a visual clue to chemical zonation, as borne out by the chemical analyses and backscatter electron images reported in the next section. The central portion of crystals, between their bases and terminations and amounting to roughly 80% of their volumes, corresponds to the new species jahnsite-(NaFeMg)."

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