Formula:Ca2Mn3+3O2(PO4)3.3H2O
Hydrated phosphate, manganese-bearing mineral,
mitridatite group,
paramorph of pararobertsite,
forms a series with arseniosiderite
Crystal System: Monoclinic
Specific gravity: 3.13 measured, 3.05 calculated
Hardness: 3½
Streak: Red-brown
Colour: Red, red-brown, deep red, bronzy brown, black
Luminescence: Not fluorescent under UV
Environments
Pegmatites
Sedimentary environments
At the Khoa Rang Kai deposit, Lamphun Province, Thailand, robertsite occurs as pore fillings, as layers and
coatings on phosphate particles and between apatite layers, and rarely as
submicron-sized crystals in apatite, in a guano-derived sedimentary phosphate
ore
(AM 74.1399-1404).
Associated minerals include carbonate-rich fluorapatite,
calcite, dolomite,
quartz and clay minerals
(HOM).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, rhodochrosite occurs as crystals to 3 cm in miarolitic
cavities, and as masses in phosphate pods. In a large mass of
rhodochrosite-siderite after
lithiophilite, millimeter-sized
sphalerite crystals associated with
perloffite have been noted. The Emmons pegmatite is an example of a highly
evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.514).
At the type locality, the Tip Top Mine, Fourmile, Custer Mining District, Custer County, South Dakota, USA,
robertsite is moderately abundant and is probably rather widespread, especially in
pegmatites where
triphylite occurs in abundance. Robertsite occurs as fine feathery
aggregates of deep red to bronzy plates, botryoidal aggregates of shiny black fibres, shiny black plates and lustrous
small wedge-shaped crystals. A spectacular specimen, measuring 6 x 7 cm2 in area, shows the mineral as broad
plates up to 5 mm across which are grouped as bunched radial aggregates along a fracture in a dense
rockbridgeite -
ferrisicklerite mass. Robertsite lines cavities in
rockbridgeite and
ferrisicklerite upon which are positioned crystals of
leucophosphite, jahnsite and
hureaulite. A late generation of this platy robertsite often is
deposited upon these minerals. Another specimen shows a dense mass of deep red-brown massive robertsite in which
cavities are lined with lustrous wedge-shaped crystals of the same.
At other localities in the Custer Mining Didistrict, at the Linwood pegmatite robertsite occurs in a paragenesis
similar to the Tip Top material, and in small amounts at the White Elephant and Gap Lode pegmatites
(AM 59.48-59).
Robertsite from the Tip Top Mine - Image
Back to Minerals