Mineralpedia Details for Jahnsite-(CaMnMg)

Named for Richard H. Jahns (1915-1983). Occur predominately in zoned pegmatites. At the type locality, the Tip Top mine near Custer South Dakota, crystals occur as light to dark brown, deep red-brown and red-orange, vitreous, short to long prismatic crystals to 7mm. Crystals are striated length-wise on the prism face [010] and are typically tabular and found as singles and subparallel groups in altered Triphylite matrix with Leucophosphite, Rockbridgeite, Hureaulite and Tavorite. Jahnsite (CaMnMg) has not been found in association with Jahnsite (NaFeMg), which occurs in Heterosite matrix.

For best reference see unpublished Thomas Campbell MS thesis, SDSM&T 1984.

Ref. Handbook of Mineralogy, Anthony et al (1995) and MSA at http://www.handbookofmineralogy.org/pdfs/jahnsite-(CaMnMg).pdf

Formula

CaMn²⁺Mg₂Fe³⁺₂(PO₄)₄(OH)₂·8H₂O 

Crystal System

Monoclinic 

Cleavage

Good, None, None 

Luster

Vitreous - Adamantine 

Color

brown, purplish brown, yellow, yellow orange, greenish yellow 

Streak

white 

Class

Monoclinic - Prismatic 

Fracture

Brittle 

Hardness

4 

WebMineral

View Jahnsite-(CaMnMg) 

Mindat

View Jahnsite-(CaMnMg) 

Special Info

Type Locality

Nut brown, striated, tabular Jahnsite (CaMnMg) to 2mm.