Potassic-fluoro-hastingsite is a member that is rare of Calcium Amphibole Subgroup of minerals that additionally includes Actinolite, Pargasite, Sadanagaite, Tremolite and Tschermakite. Potassic-fluoro-hastingsite is very rare and mineral that is obscure only discovered in a single spot in the world – the Greenwood Mine within the Town of Tuxedo, Orange County, brand new York, USA. It is rarely found in mineral collections as well as rarer as a treasure that is faceted! It really is black with vitreous luster and is barely radioactive due to its potassium content.

Potassic-fluoro-hastingsite was discovered in 2005 in the tailings dump of the Greenwood (Patterson) iron mine. The mine, located in Hudson Highlands, in Harriman State Park, Town of Tuxedo, Orange County, New York, USA, ended up being first exposed in 1838, and was a way to obtain iron during the Civil War. It was last worked in 1880.

This mineral was originally named Fluoro-potassic-hastingsite in 2009 by Marian V. Lupulescu, John Rakovan, M. Darby Dyer, George W. Robinson, and John M. Hughes because of its chemical relationship to Hastingsite. However it was renamed and redefined in 2012 by the Amphibole Subcomittee of the Commission on New Minerals, Nomenclature and Classification (CNMNC). The IMA that is brand new approved is Potassic-fluoro-hastingsite following the principle that the various prefixes to your root name are listed in your order they happen in the formula.

Chemical Formula: KCa2(Fe42+Fe3+)(Al2Si6O22)(F,OH)2
Potassium Calcium Iron Aluminum Silicate Floride Hydroxide
Molecular Weight: 947.86 gm
Composition: Potassium 4.12 % K 4.97 % K2O
Calcium 8.46 % Ca 11.83 % CaO
Magnesium 5.13 % Mg 8.50 % MgO
Aluminum 5.69 % Al 10.76 % Al2O3
Iron 17.68 % Fe 15.16 % FeO / 8.42 % Fe2O3
Silicon 17.78 % Si 38.03 % SiO2
Oxygen 37.13 % O
Fluorine 4.01 % F 4.01 % 
  –  % F -1.69 %  -O=F2
  100.00 % 100.00 % = TOTAL OXIDE


Crystallography: Monoclinic – Prismatic
Crystal Habit: Crystals prismatic; in compact aggregates to 1 cm
Twinning: None


Cleavage: Perfect on {110}, intersecting at 56°
Fracture: Concoidal, sub-concoidal
Tenacity: Brittle
Moh’s Hardness: 6.0
Density: 3.289 (g/cm3)
Luminescence: None
Radioactivity: Barely Detectable; GRapi = 58.40 (Gamma Ray American Petroleum Institute Units)


Color: Black, dark greenish black on thin edges
Transparency: Transparent to translucent
Luster: Vitreous, sub-vitreous
Refractive Index: 1.668 – 1.698  Biaxial  ( – )
Birefringence: 0.030
Dispersion: Moderate to strong; r
Pleochroism: Strong; X = bluish green; Y = greenish to brownish green; Z = blue to light blue