PEDRAS ORNAMENTAIS / ORNAMENTAL STONES / NATURSTEINE

http://www.stones-natursteine-portugal.pt/

 

 

                                                                               

Desde 1973 ao serviço das pedras ornamentais (mármores, granitos e ardósias).
Since 1973 exporting all types of Portuguese ornamental stones.
Seit 1973 im Dienst der portugiesischen Natursteine.

 

Rochas Ornamentais

1) CONCEITOS E DEFINIÇÕES

As rochas ornamentais e de revestimento, também conhecidas como pedras naturais, abrangem os tipos litológicos que são extraídos em forma de blocos ou chapas e cortados e beneficiados de diversas formas. Os campos de aplicação são inúmeros, indo de peças isoladas até o uso em massa nas edificações como revestimentos internos e externos.

Quando se trata de produção, 80% desta nos dias de hoje, é transformada em chapas e ladrilhos para revestimentos, 15% destinada as peças funerárias e os outros 5% em áreas diferenciadas. Nos revestimentos 60% são os pavimentos, 16% fachadas externas, 14% interiores e 10% são os acabamentos. Os mármores representam cerca de 45% da produção do mercado mundial, 40% são os granitos, 5% são os quartzitos e similares e 5% as ardósias.

2) TIPOLOGIA

O comércio subdivide as rochas ornamentais em, basicamente, dois grupos: granitos (rochas silicáticas) e mármores (rochas carbonáticas). Mas também há alguns outros tipos litológicos que englobam os quartzitos, os serpentinitos, os travertinos e as ardósias.

As rochas isótropas são homogéneas e mais usadas para revestimento, as rochas anisótropas são as movimentadas e geralmente utilizadas em peças isoladas. O padrão cromático é o mais levado em consideração na hora da qualificação comercial da rocha  e pode ser de três tipos : clássica, comum ou excepcional.

Os produtos obtidos através da extração de blocos e serragem de chapas que sofrem algum tipo de beneficiamento (polimento e lustro), são designados como rochas processadas especiais. E os produtos que são utilizados com as superfícies naturais em peças na calibragem, extraídos diretamente por laminação mecânica de chapas na pedreira, são chamados rochas processadas simples. As ardósias recebem um nome especifico, pois são comercializadas pela cor. Os serpentinitos tem os seus produtos comercializados sob a designação de mármores verdes.

3) NOÇÕES GERAIS DO BENEFICIAMENTO

O beneficiamento das rochas ornamentais refere-se ao desdobramento de materiais brutos, extraídos nas pedreiras em forma de blocos ou, em alguns casos específicos, em placas. A dimensão dos blocos varia de 5 a 10 m³, que são beneficiados sobretudo através da serragem (processo de corte) em chapas, por engenhos de serragem e talha blocos, para que no fim haja o acabamento para a dimensão final.

4) ACABAMENTO DE SUPERFÍCIES

O acabamento final vem logo após a serragem, o processo dá-se através de amaciamento, polimento e lustro, ou bujardagem ou  flamejamento. O amaciamento ou desbaste representa o adelgaçamento  das chapas, com a criação de superfícies planas e paralelas. O polimento conduz o desbaste fino da chapa e o fechamento dos grãos minerais, criando uma superfície lisa, opaca e mais impermeável que a face natural da mesma rocha. O lustro é aplicado no sentido de se imprimir brilho à superfície da chapa, produzido pelo espelhamento das faces dos cristais constituintes da rocha.

Os processos são realizados por cabeças abrasivas, à base de carbureto de sílico e diamante, em diferentes granulometrias (mais grosso para o amaciamento, e cada vez mais finos para o polimento e lustro final). O brilho define o resultado do polimento e do lustre, fechamento e espelhamento das chapas, podendo-se aferir o brilho através da acuidade visual ou com uso de aparelhos (glossmeter – medidores de brilho).

 

The quintessence is sawn and cut and polished and most ably transformed by rough and hard-skinned hands that shape this stones into delicate home embellishments and utilities.

Throughout history, human beings have made use of natural stone for their social and cultural manifestations. The greatest legacy we have received from our ancestors derives from their use of stone. It is through the use of natural stone that we have found out most of what we know about their way of life. 

Throughout history, human beings have made use of natural stone for their social and cultural manifestations. The greatest legacy we have received from our ancestors derives from their use of stone. It is through the use of natural stone that we have found out most of what we know about their way of life. 

 

Natural Stones are quarried out of mountains or underground from all over the world and then cut into blocks.

The various sized stone blocks are then transported to the cutting and finishing plants where the stones are cut into various sizes and shapes and then the face is finished into a polished, honed, flamed, brushed, hammered, etc. based on the customer’s specifications.

One beautiful characteristic of natural stone is that there are no two pieces of natural stone alike. Some stones may have extreme variation in color and veining from tile to tile or slab to slab.

This characteristic is common in many types of stone, and is part of the inherent beauty of using a natural product in your home or project. 


The most common Natural stones are:

Marble * Granite * Travertine * Limestone * Slate * Sandstone * Quartzite 

MARBLE
Marble is a metamorphic rock resulting from the recrystallization of limestone. Commercially, however, all calcareous rocks produced by nature and capable of taking a polish are called marbles, as are some dolomite and serpentine rocks. 

GRANITE
Granite, igneous rock of visible crystalline formation and texture. It is composed of feldspar (usually potash feldspar and oligoclase) and quartz, with a small amount of mica (biotite or muscovite) and minor accessory minerals, such as zircon, apatite, magnetite, ilmenite, and sphene. Granite is usually whitish or gray with a speckled appearance caused by the darker crystals. Potash feldspar imparts a red or flesh color to the rock. Granite crystallizes from magma that cools slowly, deep below the earth's surface. Exceptionally slow rates of cooling give rise to a very coarse-grained variety called pegmatite. Granite, along with other crystalline rocks, constitutes the foundation of the continental masses, and it is the most common intrusive rock exposed at the earth's surface. 

Although granite has been known as igneous rocks derived from, molten masses or magmas, but there is wide evidence that the origin of some granite may be attributed to regional metamorphism or preexisting rocks, rearrangement and recrystallization taking place without a liquid or molten stage. 

The specific gravity of granite ranges from 2.63 to 2.75. Its crushing strength is from 1050 to 14,000 kg per sq cm (15,000 to 20,000 lb per sq in). Granite has greater strength than sandstone, limestone, and marble and is correspondingly more difficult to quarry. It is an important building stone, the best grades being extremely resistant to weathering. 

Normally granite is classified in three different groups:

FINE GRAIN: Fine grain granites are those which the feldspar crystals average about 1/16 - 1/8" in diameter.
MEDIUM GRAIN: Medium grain granites are those in which the feldspar crystals average about 1/4" in diameter.
COARSE GRAIN: Coarse grain granites are those in which feldspar crystals average 1/2" , and greater diameter or several centimeters in maximum dimension. Coarse grain granites may have a lower density.
In recent years about 83 percent of the stone used for monuments has been granite, about 17 percent, marble.


LIMESTONE
Limestone is defined as a rock of sedimentary origin composed principally of calcium carbonate or the double carbonate of calcium and magnesium, or a combination of these two minerals. Recrystallized limestone, compact microcrystalline limestone, and travertine that are capable of taking a polish are promoted, marketed and sold as either limestone or marble, particularly in the United States. Dimension limestone is divided into three sub-classifications that describe their densities in approximate ranges, as follows:

LOW DENSITY - Limestone having a density ranging from 110 through 135/lb/ft3 (1760 through 2160 kg/m3)..
MEDIUM DENSITY - Limestone having a density greater that 135 and not greater than 160 lb/ft3 (2160 through 2560 kg/m3).
HIGH DENSITY - Limestone having a density greater than 160 lb/ft3 (2560 kg/m3).
Limestone contains a number of distinguishable natural characteristics, including calcite streaks or spots, fossils or shell formations, pit holes, reedy formations, open texture streaks, honeycomb formations, iron spots , travertine-like formations and grain formation changes. One or a combination of these characteristics will affect the texture.


SLATE
The shale from which slate originate were deposited previously on clay beds. Subsequent earth movements tilted these beds of shale, at first horizontal, and the intense metamorphism that converted these into slates folded and contracted them. Slate, then , belongs to the metamorphic group of rocks and can be defined as a fine grain rock derived from clays and shale and possessing a cleavage that permits it to be split into thin sheets.

QUARTZITE
Quartzite, common and widely distributed rock composed mainly or entirely of quartz. The compact, granular rock is a form of metamorphosed sandstone in which silica, or quartz, has been deposited between the grains of quartz of which the sandstone is essentially composed. Other minerals that may be present in small amounts in quartzite include feldspar, mica, rutile, tourmaline, and zircon. Quartzite has a smooth fracture and is found primarily among ancient rocks, such as those of the Cambrian or Precambrian system

The " Estremoz marbles" were for decades the hallmark of Portuguese Dimension Stone. Mostly economic constraints associated with a lack of knowledge regarding the physical, chemical and mechanical properties of different rocks of prescribers, created a "fashion" that focuses primarily on the homogeneity of the material don´t having in account its longevity. We note that there was no depletion of raw material; in fact almost every varieties of marble have enough reserves to sustain a mining activity for several hundred of years.
Despite its dimension, Portugal has mining units of ornamental rocks throughout its territory. In fact, the North its rich in igneous rocks, particularly granites, while the microcrystalline sedimentary calcareous are concentrated in the Maciço Calcário Estremenho. There are considerable reserves of limestone breccias in Algarve (S. Brás de Alportel – Tavira, southern Portugal) as well as nepheline syenite, unique in the World and explored in Serra de Monchique (Algarve). Schists have an extractive pole of considerable dimension in the area of Porto, registering also additional 
exploration in Alentejo, in Barrancos. 
The marbles are exploited in the province of Alentejo, southern Portugal, between Sousel and Alandroal, were can be individualized the Estremoz anticline. This main symmetrical and elliptic geological structure, orientated NW-SE and measuring some 42x8 km, is located within the greater Ossa Morena Zone geological unity in which Lower Palaeozoic marbles with ornamental interest outcrop over 27 km2.
The Estremoz anticline is the main centre for Portuguese marble exploitation and one of the most important internationally. A large number of quarries in the area greatly simplifies the access to the marbles and provide unique geological windows some of which reach some 150 m in depth.The marbles preserve the effects of the Variscan Orogeny and several structures preserved in the quarries originates beautiful aesthetic patterns that frequently are emphasized in the final applications of the marbles.
Other Palaeozoic marbles, which are less relevant, also outcrop in Alentejo (Vila Verde de Ficalho, Trigaches, Serpa, Viana do Alentejo and Escoural). In every case, the marbles occur integrated in Volcano-Sedimentary Complexes. 

Dimension Stones Productions 
Despite a relentless global economic recession, slightly contradicted by the contribution of countries like China and India, both with an increasingly evident position in the landscape of world trade in ornamental rocks, Portugal, though it has been losing market share and the difficulty of remaining competitive with some Eastern countries, Turkey and Brazil, remains firmly in a prominent position in terms of production of dimension stones (Table 1) thanks to the quality shown by the stone material and the national adaptation business. 
On the weight of the national market of Dimension Stone, marble from Estremoz, Borba and Vila Viçosa for many years were positioned in the top position in regards to extraction, processing and export, however, progressively, it has been losing ground to other ornamental rocks, specifically, the sedimentary limestone of the Maciço Calcário Estremenho, with strong development thanks to the Chinese market and also to the clusters which have gradually been gaining market and are now a reality on the production of kitchen worktops.