Abrasivegrade garnet study: Mitchell Co', North Carolina

1
Abrasive-grade garnet study Mitchell Co., North
Carolina
EVALUATION OF THE BAKERSVILLE ECLOGITE, MITCHELL
COUNTY, NORTH CAROLINA FOR ITS SUITABILITY AS AN
ABRASIVE-GRADE GARNET MENSAH-BINEY, Robert,
BALDWIN, Robert, North Carolina State
Universitys Minerals Research Laboratory, 180
Coxe Ave., Asheville, NC 28801, REID, Jeffrey C.,
North Geological Survey, 1612 Mail Service
Center, Raleigh, NC 27699-1612. A heavy mineral
garnet concentrate, suitable as an abrasive, was
produced from a 2 kg sample of garnet-rich
material from the Bakersville eclogite, Mitchell
County, North Carolina. The bench scale
laboratory process consisted of crushing,
grinding, and gravity separation to separate the
heavy minerals from the light minerals. The
sample was ground to pass US 30-mesh and the
minus 30-mesh fraction was subjected to sink
float separation using a heavy liquid with
density of 2.96 g/cc. The sink fraction (heavy
mineral concentrate) composed mainly of garnet
was subjected to mineralogical evaluation to
determine garnet grade. The results of the heavy
liquid separation process indicated that about
85 by weight of the original material reported
as heavy mineral concentrate. The garnets are
red, subhedral, and average 2 mm in diameter.
Microprobe compositions are Py32Alm47Grs22Spslt1
rims and Py27Alm49Grs24Spslt1 cores (Willard
and Adams, 1994)1. 1Reference - Willard, R.A.,
and Adams, M.G., 1994, Earth and Planetary
Science Letters, v.123, pp. 61-70.
(Left - top) -- Location map showing location in
North Carolina and field trip stop 1-5 from the
1997 Carolina Geological Society field trip
guidebook (Stewart and others). Sample for this
study not collected from this specific locality.
(Left) -- Typical garnet separated from the Lick
Ridge Eclogite. Scale shows grain phi size and mm
(top scale rule markings)
(Above) -- Photograph of garnet-rich rock
(eclogite) used in this mineral separation
study. (Left) Geologic map of eclogite-bearing
terrane northeast of Bakersville, N. C. Geologic
map from Adams (1995). Explanation Zas AMS
Ashe Metamorphic Suite metasedimentary rocks,
predominantly metapelite and metagraywacke. Zaa
AMS amphibolite. Consists of amphibolite and
garnet amphibolite. Zae AMS eclogite. Consists
of eclogite and retrogressed ecologite. Ymg
Laurentian basement rock. Mafic gneiss and felsic
gneiss. mz Mixed rock mylonite zone. Consists
of mylonite derived from rocks of hanging wall
and footwall.

• PROCESSING STEPS FOR GARNET SEPARATION
• Crush raw ore through a jaw crusher to ¾
• Re-crush product in Chipmunk to ¼
• Crush in Roll Crusher to 10 US Standard mesh
• Grind in rod mill to 30 US Standard Mesh
• Screen ground product on 200 US Standard Mesh to
  remove 200 mesh fraction
• Attrition scrub 30 x 200 mesh fraction at 70
  solids for 5.0 minutes with NaOH
• Deslime on 200 mesh sieve to remove 200 mesh
  slime
• Perform heavy liquid separation (sink-float)
  using 2.96 g/cc liquid
• Sink product is garnet and other heavy minerals
  possibly magnetite
• Perform particle size analysis on the sink
  fraction (garnet)

(Above) -- SEM backscatter image of typical
garnet from the Lick Ridge Eclogite. Inclusions
are predominantly quartz (dark) and rutile
(white). Subtle zoning in the garnet is evident
in the shades of gray with a lighter
inclusion-rich core and darker mantle. Sample BAK
11509 Courtesy of Dr. Kevin Stewart UNC-CH.
(Above) -- Detail of garnet-rich rock (ecolgite)
as shown in the poster panel above. The red
appearing mineral is garnet.
Base map is from the Bakersville 7.5-minute
quadrangle.
Dry particle size analysis of garnet (200 mesh
sinks)
Results (continued)
Results (continued)

• Selected information about other garnet resources
  in western North Carolina
• North Carolina Geological Survey
  (www.geology.enr.state.nc.us ? Minerals
  Information)
• Mineral file contents Archdale office (see
  garnet)
• Mineral file contents Asheville regional office
  (see garnet)
• Carolina Geological Society (www.carolinageologica
  lsociety.org)
• Stewart, K.G., Adams, M.G., and Trupe, C,H.,
  editors, 1997, Paleozoic structure, metamorphism,
  and tectonics of the Blue Ridge of western North
  Carolina
• U. S. Geological Survey
• McFaul, E.J., Mason, G.T., Jr., Ferguson, W. B.,
  and Lipin, R.R., 2000, U. S. Geological Survey
  Mineral Databases MRDS and MASMILS, U. S.
  Geological Survey Digital Data Series DDS-52 (two
  disks).

Cumulative weight percent retained curve for
garnet (200 mesh sink).
Bar chart of particle size analysis of garnet.
Table of particle size analysis of garnet.