Mineralpedia Details for Montgomeryite

Named after Arthur Montgomery (1909-1999) who was an American mineralogist and Professor of Geology at Lafayette College in Pennsylvania, USA and was the first to collect specimens of the mineral. It is an uncommon mineral that occurs in pegmatites including here in the Black Hills at the Tip Top, White Elephant, Wharf, and Etta mines in South Dakota, USA and in nodules in phosphate rich sedimentary deposits. localities include Brazil, Australia, Portugal, Spain, and Namibia, among others. At each deposit, crystals are consistently lath-like and flattened on {010} and elongated and striated along [001] with inclined terminations. Colors range from  colorless, white, light yellow, mint green, and pink to deep red.

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

Formula

Ca₄MgAl₄(PO₄)₆(OH)₄·12H₂O 

Crystal System

Monoclinic 

Cleavage

Perfect, Indistinct, None 

Luster

Vitreous (Glassy) 

Color

colorless, yellowish, white, green, red 

Streak

white 

Class

Monoclinic - Prismatic 

Hardness

4 

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Deep red lath-like tufts of Montgomeryite crystals. Individual crystals are from 0.5 to 1mm.

Yellowish green lath-shaped or bladed crystals less that 1cm with inclined terminations. This occurence, like many other Montgomeryite occurences, show the consistency of crystal habits of Montgomeryite, much like the Type Locality at Fairfield, Utah as described by Esper Larsen. The b {010} prism face is broad and with many vicinal faces giving the appearance of striations. The terminations are pyramidal with a short and long sides of the p{111}. See also AM Larsen (Am. Min., 25, 315, 1940).