This paper aims at original elevation of clay mineral occurrences with focus on their utilization. The area of the research is the South of Portugal with 1) Algarve coastal plain from tertiary and quaternary sediments grading into Mesozoic lime-stones and marls; 2) the Algarve mountain range with magmatic plutonic rocks overlain by Mesozoic consolidated sediments; to the North 3) the Baixo Alentejo follows a sedimentary basin filled with tertiary sediments covering a Palaeozoic bottom; the last area 4) Alto Alentejo is again dominated by plutonic and metamorphic rocks which have been peniplainated during the tertiary period. In all areas we find terrestrial soils placed on terraces of the younger river systems. Temperature regime is mesic, the moisture regime is ustic with a temperate moist winter period and a hot dry summer time. In general, rock minerals are transformed into soil minerals under the influence of climate, relief and organisms. With terrestrial soil the latter factors are constant. Therefore, the hypothesis is that the soil minerals are the function of time and rock. All clay mineral determinations of our archive taken through the last 25 years have been screened taking these two factors into account. It was soon realized that under specific conditions we do find well developed clay minerals without or with low impurities. Illite is completely inherited from Palaeozoic micaschists in the Haplic Luvisol. Smectite seems to be a very typical clay mineral, which is mainly related to the climatic conditions of the Mediterranean and can be found on different rocks. On the other hand, Kaolinite can be also found extremely well developed, but only in rather old generally Pliocene high plateaus, but then also from different rocks (marble, sand stone). Formation of Kaolinite needs export of silica. During the research in the Algarve and Alentejo area three rather rare minerals have been detected. The first was Talc, the second Palygorscite and the third Opal. The Talc seems to be specific development through weathering of calcium-silicate rocks (Skarn), a lot of magnesium with free silicate are deliberated. The formation will be long lasting, but could be also under modern conditions. This is different with Palygorscite. The Palygorscite is cementing in mid-tertiary fluvial sediments, which have been weathering under alkaline conditions. Under similar conditions, mainly in the influence of ground water, mobile silica is precipitated as cement between coarse grains. The Opal is found in general in slightly acid soil conditions. From Tab. 1 one may derive some conclusions or clear tendencies of clay mineral development:Kaolinite and Opal are found from acidic rocks and land surfaces older than Pliocene. Smectite and Palygorscite are occurring on basic rocks which are older than Pliocene or Miocene. Kaolinite and Talc are found on rather old karst land surfaces. Illite (mica clay) is bound to acidic micaschists in modern soils. Again Smectite can be formed under alkaline conditions during the Holocene class. Finally, Smectite may be formed in soils with impeded drainage during Holocene or Pleistocene. From the minerals analyzed Palygorscite seems to be worthwhile to be mined. Talc and Opal would need a lot of pre-treatment, while Kaolinite and Smectite can be found in places with sufficient amounts to be excavated.

Parent material	Kaolinite	Illite	Expandite	Chlorite	Palygorscite	Talc	Opal
Angaben in %
Cover Sediment	14	14	72	0	0	0	0
Ranás	76	12	9	3	0	0	0
Tertiary Sand	14	4	78	0	0	0	84*
Tertiary Marl	4	9	80	<1	6	0	0
Other Marls	23	38	38	0	0	0	0
Limestone	36	37	26	2	0	0	0
Marble	67	18	13	1	0	0	0
Micaschiests	38	32	21	0	0	0	0
Smectitic Schists	17	15	68	0	0	0	0
Skarn	55	13	18	0	0	14	0
Granodiorite 1	42	20	27	7	0	0	0
Granodiorite 2	16	17	56	11	0	0	0
Gneiss	6	8	86	0	0	0	0

Tab. 1: Content (average) of clay mineral groups

in soils from different rocks and land surfaces

in Southern Portugal