Skip to Main Content
Skip Nav Destination
GEOREF RECORD

Windmountainite, Fe (super 3+) (sub 2) Mg (sub 2) (sub 2) Si (sub 8) O (sub 20) (OH) (sub 2) (H (sub 2) O) (sub 4) .4H (sub 2) O; a new modulated, layered Fe (super 3+) -Mg-silicate-hydrate from Wind Mountain, New Mexico; characterization and origin, with comments on the classification of palygorskite-group minerals

Derek D. Leung and Andrew M. McDonald
Windmountainite, Fe (super 3+) (sub 2) Mg (sub 2) (sub 2) Si (sub 8) O (sub 20) (OH) (sub 2) (H (sub 2) O) (sub 4) .4H (sub 2) O; a new modulated, layered Fe (super 3+) -Mg-silicate-hydrate from Wind Mountain, New Mexico; characterization and origin, with comments on the classification of palygorskite-group minerals
The Canadian Mineralogist (July 2020) 58 (4): 477-509

Abstract

Windmountainite, ideally Fe (super 3+) (sub 2) Mg (sub 22) Si (sub 8) O (sub 20) (OH) (sub 2) (H (sub 2) O) (sub 4) .4H (sub 2) O, is a new mineral species and member of the palygorskite group discovered as orange-brown, radiating aggregates that commonly fill vesicles (average 1.5X2.5 mm) within a phonolite dike at Wind Mountain, Otero County, New Mexico, USA. The mineral develops as tightly bound bundles (up to 0.02X6 mm) of acicular to bladed crystals that are elongate on [001] and flattened on the pinacoid {010}. Associated minerals include albite, aegirine, fluorapophyllite-(K), natrolite, neotocite, and montmorillonite, the last of these being observed to replace primary windmountainite. It has a dull luster, silky in aggregates, is translucent and has an orange-brown streak. It does not fluoresce under short-, medium-, or long-wave ultraviolet radiation. Windmountainite is brittle with a splintery fracture and has two good cleavages (predicted) on {110}, an estimated hardness of 2, a calculated density of 2.51 g/cm (super 3) , and a calculated n (sub avg) of 1.593. A total of n=30 EMPA (WDS) analyses from six grains yielded an average of (wt.%): Na (sub 2) O 0.08, MgO 3.47, Al (sub 2) O (sub 3) 1.15, SiO (sub 2) 49.76, Cl 0.07, K (sub 2) O 0.40, CaO 0.68, TiO (sub 2) 0.30, MnO 5.64, Fe (sub 2) O (sub 3) 20.17, H (sub 2) O (calc.) 16.59, O=Cl -0.02, total 98.29. The empirical formula [based on Sigma (T1, T2, M2, M3)=12 cations pfu, excluding Ca, K, and Na] is: ( (sub 0.78) Ca (sub 0.12) K (sub 0.08) Na (sub 0.02) ) (sub Sigma 1.00) (Fe (super 3+) (sub 1.93) Al (sub 0.04) Ti (sub 0.02) ) (sub Sigma 1.99) (Mg (sub 0.81) Mn (super 2+) (sub 0.75) Fe (super 3+) (sub 0.44) ) (sub Sigma 2.002) (Si (sub 7.81) Al (sub 0.17) Ti (sub 0.01) Fe (super 3+) (sub 0.01) ) (sub Sigma 8.00) O (sub 20) [(OH) (sub 1.98) Cl (sub 0.02) ] (sub Sigma 2.00) [(H (sub 2) O) (sub 3.38) (OH) (sub 0.62) ] (sub Sigma 4.00) .4H (sub 2) O, yielding the simplified formula, (sub ) Fe (super 3+) (sub 2) Mg (sub 22) Si (sub 8) O (sub 2) 0(OH) (sub 2) (H (sub 2) O) (sub 4) .4H (sub 2) O. The predominance of Fe (super 3+) is based on color, results from the crystal-structure refinement, the crystal-chemistry of palygorskite-group minerals, the association with Fe (super 3+) -dominant minerals, and considerations regarding the late-stage geochemical evolution of agpaitic rocks. The presence of H (sub 2) O and OH was determined based on results from the refined crystal structure and Fourier-transform infrared spectroscopy. Windmountainite crystallizes in the space group C2/m with a 13.759(3), b 17.911(4), c 5.274(1) Aa, beta 106.44(3) degrees , V 1246.6(1) Aa3, and Z=2. The seven strongest powder X-ray diffraction lines are [d in Aa (I), (hkl)]: 10.592 (100) (110), 5.453 (16) (130), 4.484 (19) (040), 4.173 (28) (221), 3.319 (53) (221, 400), 2.652 (30)(440, 351), 2.530 (27)(002, 261). The crystal structure was determined from single-crystal X-ray diffraction data and refined to R=4.01% and wR (sub 2) =10.70% using data from 902 reflections (F (sub o) >4sigma F (sub o) ). It is based on sheets of inverted double chains of SiO (sub 4) tetrahedra that sandwich ribbons of Mphi (sub 6) octahedra (phi =O, OH, H (sub 2) O, Cl), giving rise to large channels ( approximately 6.5X9 Aa) that are occupied by loosely held H (sub 2) O groups. A modified classification of the palygorskite group [general crystal-chemical formula M1M2 (sub 2) M3 (sub 2) M4 (sub 2) T1 (sub 4) T2 (sub 4) O (sub 20) (OH) (sub 2) (H (sub 2) O,OH) (sub 4) .W] is proposed based on the occupants of the four M sites. Within this scheme, windmountainite is the (sub ) -Fe (super 3+) -Mg- (sub ) member. The palygorskite group includes six members: palygorskite (monoclinic and orthorhombic polytypes), yofortierite, tuperssuatsiaite, raite, windhoekite, and windmountainite. Windmountainite is considered to have formed from late-stage fluids that were alkaline, oxidized, and rich in both Fe (super 3+) and H (sub 2) O; high aH (sub 2) O conditions are reflective of abundant, hydrated feldspathoids (natrolite and analcime) forming as primary rock-forming minerals in the phonolite at Wind Mountain.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 58
Serial Issue: 4
Title: Windmountainite, Fe (super 3+) (sub 2) Mg (sub 2) (sub 2) Si (sub 8) O (sub 20) (OH) (sub 2) (H (sub 2) O) (sub 4) .4H (sub 2) O; a new modulated, layered Fe (super 3+) -Mg-silicate-hydrate from Wind Mountain, New Mexico; characterization and origin, with comments on the classification of palygorskite-group minerals
Affiliation: Laurentian University, Harquail School of Earth Sciences, Sudbury, ON, Canada
Pages: 477-509
Published: 202007
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 88
Accession Number: 2021-007967
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 14 tables, sketch map
N32°00'00" - N33°22'60", W106°22'60" - W104°52'00"
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2021
Close Modal

or Create an Account

Close Modal
Close Modal