Gneiss
Gneiss
Gneiss (pronounced "nice") is a metamorphic rock consisting mostly of quartz and feldspar and showing distinct layering or banding. The layering of a gneiss may be weak or well-developed and consists of varying concentrations of biotite, garnet, hornblende, mica, and other minerals . These structures do not record a layered deposition process but arise from preferential recrystallization along flow or stress lines during metamorphosis of the parent rock (protolith).
The gneisses are a very varied group, including both igneous rocks and metamorphosed sedimentary rocks , and may be categorized as quartzofeldspathic, pelitic, calcarous, or hornblende gneiss.
Quartzofeldspathic gneiss forms by metamorphosis of either silicic igneous rocks such as granite , rhyolite , and rhyolitic tuff—or silicic sedimentary rocks such as sandstone . Quartzofeldspathic gneiss containing eye-shaped feldspar crystals is termed augen gneiss after the German augen (eyes).
Pelitic gneiss is formed by metamorphosis of clay-textured sedimentary rocks, particularly those rich in iron .
Calcareous gneiss contains calcite (CaCO3). It is formed by metamorphosis of limestones and dolomites containing large fractions of sand and clay . Calcareous gneisses with large fractions of calcite blur conceptually with the marbles (metamorphosed limestones).
Hornblende gneiss contains a large fraction of hornblende in addition to its quartz and feldspar.
The gneisses can be alternatively categorized simply as orthogneisses and paragneisses. The former are metamorphosed from igneous protoliths and the latter from sedimentary protoliths.
The gneisses and schists are closely related. Both are metamorphosed igneous or sedimentary rocks showing foliation or layering. The difference is primarily one of degree; schists are less coarsely crystallized and more prone to cleave into flakes or slabs. Gneisses represent a higher grade of metamorphosis—more thorough melting—and are distinguished by their coarser texture and their resistance to cleavage.
See also Migmatite
Cite this article
Pick a style below, and copy the text for your bibliography.
|
Preservationists work to save historic buildings at 156-year-old Fort Sam Houston.(Knight Ridder Newspapers)
Newspaper article from: Knight Ridder/Tribune News Service; 7/19/2001; ; 700+ words
; ...Indians and Mexicans have faded, but Fort Sam Houston, a 3,100-acre post just a mile...historic landmarks in Texas, Fort Sam has the largest collection of historic...that those same soldiers roll into Fort Sam Houston, some on their first assignment...
|
|
Fort Sam Houston Military Base in Texas Launches $1.37 Billion Housing Project.
Newspaper article from: Knight Ridder/Tribune Business News; 3/18/2004; 700+ words
; ...Business News Mar. 18--Fort Sam Houston got a fresh start Wednesday...development plan for Fort Sam. The post hopes to build...Co., will finance its Fort Sam construction and renovation...Groundbreaking for the first Fort Sam construction project is...
|
|
FORT SAM HOUSTON OBSERVES SUICIDE PREVENTION WEEK
News Wire article from: US Fed News Service, Including US State News; 9/11/2008; 700+ words
; ...following news release: Soldiers and civilians at Fort Sam Houston observed Suicide Awareness Prevention Week Sept. 9...of all who are working to prevent suicide. The Fort Sam Houston post community participated in the observance by having...
|
|
Fort Sam Houston employs team effort to accomplish Army Medical Action Plan.(United States Military Section)
Magazine article from: The Exceptional Parent; 9/1/2007; ; 700+ words
; ...Medical Command and Walter Reed Army Medical Center. Fort Sam Houston is one example of many Army installations that are...successful AMAP. The Army Medical Action Plan at Fort Sam Houston rocks! That is the message delivered to the Brooke...
|
|
TEXAS NATIONAL GUARD, FORT SAM HOUSTON CONTINUE RITA RELIEF EFFORTS
Transcript from: Regulatory Intelligence Data; 9/28/2005; ; 640 words
; ...efforts for areas affected by Hurricane Rita, and Fort Sam Houston, in San Antonio, has been providing essential support...both National Guard fuel distribution sites. Fort Sam Houston has been carrying out several support missions to facilitate...
|
|
Fort Sam, The Story of Fort Sam Houston, Texas
Magazine article from: Military Images; 9/1/2004; ; 362 words
; Fort Sam, The Story of Fort Sam Houston, Texas, by Eldon Cagle Jr. 103 pages, 176 illustrations. Maverick...78209, 2003. $16.95 soft cover. Completed in 1879, Ft. Sam Houston, located in San Antonio, Texas, has a long and illustrious...
|
|
|
FORT SAM HOUSTON GROWTH MANAGEMENT PLAN STIMULATES CITY'S EAST SIDE
News Wire article from: US Fed News Service, Including US State News; 8/22/2008; 625 words
; ...Growth Management plan surrounding Fort Sam Houston. Starting Monday, August 25 through...approximately 12,500 people to the Fort Sam roles. Of that number, approximately...of neighborhoods around Fort Sam Houston and recommends the establishment...
|
|
Army sex scandal spreads to third post Allegations of improprieties surface at Fort Sam Houston
Newspaper article from: The Milwaukee Journal Sentinel; 11/14/1996; 700+ words
; Drill sergeants and classroom instructors at Fort Sam Houston in San Antonio routinely took medical trainees on chartered...post. Details of what one Army captain called Fort Sam Houston's "Mexico trip episode" come in the wake of rape...
|
|
United States: Fort Sam Houston slated to receive nearly $30 million in additional funding.
News Wire article from: TendersInfo; 6/22/2008; 700+ words
; ...Antonio's congressional delegation announced that Fort Sam Houston will receive $28 million as part of an $863 million...ongoing construction and health care projects at Fort Sam Houston, which is undergoing a transformation under BRAC into...
|
|
DTI Opens Offices on Fort Sam Houston.
Business Wire; 10/4/2006; 503 words
; SAN ANTONIO -- Fort Sam Houston continues to grow as it welcomes The Defense Transformation Institute...such close proximity to one of our primary collaborators at Fort Sam Houston. As the Post prepares for such major transformation over the next...
|
|
Fort Sam Houston
Book article from: The Columbia Encyclopedia, Sixth Edition
Fort Sam Houston U.S. army base, 3,300 acres (1,335 hectares), S Tex., in...military post that was constructed from 1876 to 1890 and named for Gen. Sam Houston. The famous Brooke Army Medical Center, the major training center for...
|
|
Bases, Military: Development of
Book article from: The Oxford Companion to American Military History
...fortifications such as Fort Jay (1794) on Governors...School was established, Fort Leavenworth, Kansas...quartered in wooden and adobe forts dotting the West and...the most famous were Fort Bliss, Texas (established...Wyoming (1849); Fort Sam Houston, Texas (1876); and...
|
|
San Antonio: Recreation
Encyclopedia entry from: Cities of the United States
...advance reservations are advised at all posts except Fort Sam Houston, which is open to the public without restriction...Established in 1876 at its present location, historic Fort Sam Houston was the site of the first military airplane flight...
|
|
|
San Antonio: History
Encyclopedia entry from: Cities of the United States
...Roman Catholicism. A presidio, or fort, was established near each mission...Remember the Alamo!" — Sam Houston's Texans defeated Santa Anna at...type aircraft, flew over Fort Sam Houston and marked the debut of military...
|
|
Cisneros, Marc: 1939—: Army General, College President, Chief Executive Officer
Book article from: Contemporary Hispanic Biography
...Cisneros was a battalion commander at Fort Hood, Texas, and an artillery commander...commander, 1963-65; U.S. Army, Fort Hood, TX, battalion commander...1992-94; Fifth U.S. Army at Fort Sam Houston, TX, commanding general, 1994...
|
