RockHoundBlog

Almandine – Mineral of the day

Filed under: Mineral of the day — Gary February 21, 2010 @ 1:55 pm

Almandine, also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands. Almandine is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe3Al2(SiO4)3. Magnesium substitutes for the iron with increasingly pyrope-rich composition.

almandine

almandine

Almandine occurs rather abundantly in the gem-gravels of Sri Lanka, whence it has sometimes been called Ceylon-ruby. When the color inclines to a violet tint, the stone is often called Syrian garnet, a name said to be taken from Syriam, an ancient town of Pegu. Large deposits of fine almandine-garnets were found, some years ago, in the Northern Territory of Australia, and were at first taken for rubies and thus they were known in trade for some time afterwards as Australian rubies.

Almandine

Almandine is widely distributed. Fine rhombic dodecahedra occur in the schistose rocks of the Zillertal, in Tyrol, and are sometimes cut and polished. An almandine in which the ferrous oxide is replaced partly by magnesia is found at Luisenfeld in German East Africa. In the United States there are many localities which yield almandine. Fine crystals of almandine embedded in mica-schist occur near Fort Wrangell in Alaska. The coarse varieties of almandine are often crushed for use as an abrasive agent.

TUFA – Rock Of The Day

Filed under: Mineral of the day — Gary May 23, 2009 @ 9:41 pm
Tufa towers at Mono Lake, California.

Tufa towers at Mono Lake, California.

Tufa is a soft, friable and porous calcite rock. It is a calcium carbonate (CaCO3) deposit that forms by chemical/biological precipitation from bodies of water with a high dissolved calcium content. Calcareous tufa is not to be confused with tuff, a hard volcanic rock that is also sometimes called tufa.

Tufa forming the Trona Pinnacles, California.

Tufa forming the Trona Pinnacles, California.

Tufa deposition occurs in seven known ways:

  1. Mechanical precipitation by wave action against the shore. This form of tufa can be useful for identifying the shoreline of extinct lakes (for example in the Lake Lahontan region).
  2. Precipitation from supersaturated hot spring water entering cooler lake water.
  3. Precipitation in lake bottom sediments which are fed by hot springs from below.
  4. Precipitation from calcium-bearing spring water flowing into an alkaline lake.
  5. Precipitation throughout a lake as the lake water evaporates, leaving the lake supersaturated in calcium.
  6. Through the agency of algae. Microbial influence is often vital to tufa precipitation and may be involved in the other methods listed.
  7. Precipitation from cold water springs (for example in the foothills of the Rocky Mountains near Hinton, Alberta).

Tufa is common in many parts of the world. There are some prominent towers of tufa at Mono Lake and Trona Pinnacles in California, USA, formed by the fourth method mentioned above whilst submerged and subsequently exposed by falling water levels. Tufa is also common in Armenia and Great Britain.

Jasper – mineral of the day

Filed under: Mineral of the day,regular postings — Gary January 16, 2008 @ 10:05 am

Jasper is an opaque, impure variety of silica, usually red, yellow or brown in color. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and at one time for snuff boxes. When the colors are in stripes or bands, it is called striped or banded jasper. Jaspilite is a banded iron formation rock that often has distinctive bands of jasper. Jasper is basically chert which owes its red color to iron(III) inclusions.

jasper

Etymology and historical/mythical usage

The name means “spotted stone”, and is derived from Anglo-French jaspre, from Old French jaspe, from Latin iaspidem, the accusative of iaspis, from Greek iaspis, via a Semitic language (cf. Hebrew yashepheh, Akkadian yashupu), ultimately from Persian yashp.

The word yashepheh in the Masoretic text of Exodus 28:20, referring to a stone in the Hoshen, is thus reflected in the Septuagint by the word Iaspis, and usually translated into English as Jasper. Despite the most common form of Jasper being red, scholars think that the yashepheh here actually refers to a green form of Jasper – which was very rare, and so highly prized; the Greeks used Iaspis to refer to the green form, while the red form simply fell under the term Sard – which just means red. Rebbenu Bachya argues that this stone represents the tribe of Benjamin, but there is actually a wide range of views among traditional sources about which tribe the stone refers to.

It is described in the Book of Revelation (21:11) as follows: “It shone with the glory of God, and its brilliance was like that of a very precious jewel, like a jasper, clear as crystal.”

Types of jasper

Jasper can appear as an opaque rock of shades of red due to mineral impurities. Patterns can arise from the formation process and from flow patterns in the sediment or volcanic ash that was saturated with silica to form jasper, yielding bands or swirls in the rock.

Jasper may be permeated by dendritic minerals providing the appearance of vegetative growths. The jasper may have been fractured and/or distorted after formation, later rebonding into discontinuous patterns or filling with another material. Heat or environmental factors may have created surface rinds (such as varnish) or interior stresses leading to fracturing.

A brown jasper that occurs as nodules in the Libyan desert and in the Nile valley is known as Egyptian jasper or Egyptian pebble.

Picture jaspers simultaneously exhibit several of these variations (such as banding, flow patterns, dendrites or color variations) resulting in what appear to be scenes or images in a cut section. Spherical flow patterns produce a distinctive orbicular appearance. Complex mixes of impurities produce color variations. Healed fractures produce brecciated jasper. Examples of this can be seen at Llanddwyn Island.

deschutese_jasper_22

The history of Biggs Jasper, DESCHUTES PICTURE JASPER, Oregon.

Thanks wikipedia

gary-


Mineral/rock of the day- Serpentine -California’s state rock

Filed under: Mineral of the day,regular postings — Gary May 20, 2007 @ 9:42 pm

SerpentineSerpentine_2Overview

Serpentine is said to owe its name either to its serpent-like colours and patterns or from an old belief that the stones were effective protection from snake bites. They have their origins in metamorphic alterations of peridotite and pyroxene. Serpentines may also pseudomorphously replace other magnesium silicates. Alterations may be incomplete, causing physical properties of serpentines to vary widely. Where they form a significant part of the land surface, the soil is unusually high in clay.

Antigorite is the polymorph of serpentine that most commonly forms during metamorphism of wet ultramafic rocks and is stable at the highest temperatures — to over 600°C at depths of 60 km or so. In contrast, lizardite and chrysotile typically form near the Earth’s surface and break down at relatively low temperatures, probably well below 400°C. It has been suggested that chrysotile is never stable relative to either of the other two serpentine polymorphs.

Samples of the oceanic crust and uppermost mantle from ocean basins document that ultramafic rocks there commonly contain abundant serpentine. Antigorite contains water in its structure, about 13 percent by weight. Hence, antigorite may play an important role in the transport of water into the earth in subduction zones and in the subsequent release of water to create magmas in island arcs, and some of the water may be carried to yet greater depths.

(more…)

Zoisite- mineral of the day

Filed under: Mineral of the day,regular postings — Gary March 27, 2007 @ 7:53 pm

ZoisiteZoisite_4Zoisite_3Zoisite_rough

Zoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Zoisite is named after the Slovene scientist Baron Sigmund Zois von Edelstein (Žiga Zois), who realized that this was an unknown mineral when it was brought to him by the mineral dealer Simon Prešern, who had discovered it in the Saualpe mountains (Svinška planina) of Carinthia in 1805. Zoisite was first known as saualpite, after its type locality. Transparent material is fashioned into gemstones while translucent-to-opaque material is usually carved into sculptural works. The latter is sometimes shot through with ruby crystals, which are completely opaque and unsuited to use as gems, yet are well colored and contrast strikingly against the green matrix of the zoisite.

(more…)

Tanzanite- December’s birthstone

Filed under: Mineral of the day,regular postings — Gary @ 7:46 pm

TanzaniteTanzanite_gemTanzanite_rough_2

Tanzanite is the blue/purple variety of the mineral zoisite (Ca2(Al. OH)Al2(SiO4)3) discovered in the Meralani Hills of northern Tanzania in 1967, near the city of Arusha. It is a popular and valuable gemstone when cut, although its durability is somewhat lacking; its tendency to break sometimes precludes appropriate use as a ring stone. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet, and sage-green depending on crystal orientation. However, most tanzanite is subjected to artificial heat treatment to improve its colour: this significantly subdues its trichroism.

Background

On July 7, 1967, Manuel de Souza, a Goan tailor and part-time gold prospector living in Arusha (Tanzania) found transparent fragments of vivid blue and blue & purple gem crystals on a ridge near Mererani, some 40 km southeast of Arusha.

(more…)

Jade is the gem name for mineral aggregates composed of either or both of two different minerals, Jadeite and Nephrite.

Filed under: Mineral of the day,regular postings — Gary March 23, 2007 @ 11:15 pm

jade rough_jadequartz_jade_rough

An ornamental stone, jade is applied to two different rocks that are made up of different silicate minerals. Nephrite jade consists of the calcium- and magnesium-rich amphibole mineral actinolite (aggregates of which also make up one form of asbestos). The rock called jadeitite consists almost entirely of jadeite, a sodium- and aluminium-rich pyroxene.

The English word ‘jade’ is derived from the Spanish term piedra de ijada (first recorded in 1565) or ‘loin stone’, from its reputed efficacy in curing ailments of the loins and kidneys. ‘Nephrite’ is derived from lapis nephriticus, the Latin version of the Spanish piedra de ijada.

(more…)

azurite – mineral of the day

Filed under: Mineral of the day,regular postings — Gary March 12, 2007 @ 11:07 pm

azurite_2azurite_chinaazurite_3
Azurite is a soft, deep blue copper mineral produced by weathering of copper ore deposits. It is also known as Chessylite after the Chessy-les-Mines[1] near Lyon, France, where striking specimens have been found. The mineral has been known since ancient times, and was mentioned in Pliny the Elder’s Natural History under the Greek name kuanos (“deep blue,” root of English cyan) and the Latin name caeruleum The blue of azurite is exceptionally deep and clear, and for that reason the mineral has tended to be associated since antiquity with the deep blue color of low-humidity desert and winter skies. The modern English name of the mineral reflects this association, since both azurite and azure are derived via Arabic from the Persian lazhward, an area known for its deposits of another deep blue stone, lapis lazuli (“stone of lazhward”).

Mineralogy

Azurite crystals are monoclinic, and when large enough to be seen they appear as dark blue prismatic crystals. Azurite specimens are typically massive to nodular, and are often stalactitic in form. Specimens tend to lighten in color over time due to weathering of the specimen surface into malachite. Azurite is soft, with a Mohs hardness of only 3.5 to 4. The specific gravity of azurite is 3.77 to 3.89. Azurite is destroyed by heat, losing carbon dioxide and water to form black, powdery copper(II) oxide.

(more…)

mineral of the day Chabazite or Chabazite-ca

Filed under: Mineral of the day,regular postings — Gary March 3, 2007 @ 11:45 pm

Chabazite_2 Intense color, displaying well on its thin matrix.
The specimen has been reviewed, photographed and published as a new discovery in the magazine ‘Mineralogical Record’ on page 479 in the volume 37, number 5.
Imilchil, Anti-Atlas  Morocco (2006)

Chabazite_ca Individual crystals, isolated and with excellent luster and color.
This specimen was photographed and published as a novelty at the 2006 Ste Marie Show in the magazine ‘Le Règne Minéral’ number 70, page 41.
Imilchil, Anti-Atlas  Morocco (2006)

Chabazite is a tectosilicate mineral of the zeolite group with formula: (Ca,Na2,K2,Mg)Al2Si4O12·6H2O. Recognized varieties include Chabazite-Ca, Chabazite-K, Chabazite-Na, and Chabazite-Sr depending on the prominence of the indicated cation.

Chabazite crystallizes in the trigonal crystal system with typically rhombohedral shaped crystals that are pseudo-cubic. The crystals are typically twinned, and both contact twinning and penetration twinning may be observed. They may be colorless, white, orange, brown, pink, green, or yellow. The hardness ranges from 3 to 5 and the specific gravity from 2.0 to 2.2. The luster is vitreous.

(more…)

Rhodochrosite- mineral of the day

Filed under: Mineral of the day,regular postings — Gary March 1, 2007 @ 11:09 am

Rhodochrosite is a manganese carbonate mineral with chemical composition MnCO3.

Rhodochrosite_Crystal Rhodochrosite, from the Sweet Home Mine, Colorado, private collection.

Rhodochrosite_PinkTypical pink form of Rhodochrosite with Quartz, from the Silverton, Colorado, private collection.

Alma_King_rhodochrosite“Alma King”, largest known rhodochrosite crystal, Denver Museum of Nature and Science

In its (rare) pure form, it is typically a rose-red colour, but impure specimens can be shades of pink to pale brown. The streak is white. Its Mohs hardness varies between 3.5 and 4. Its specific gravity is 3.5 to 3.7. It crystallizes in the trigonal system. The cleavage is typical rhombohedral carbonate cleavage in three directions. Crystal twinning often is present. It is transparent to translucent with refractive indices of nω=1.814 – 1.816 nε=1.596 – 1.598. It is often confused with the manganese silicate, rhodonite, but is distinctly softer.

(more…)