I think "prafos" would be "silty" in English here, and thus that your whole phrase would be "silty clays and clayey silts".

In the table in the link below, "silturi" are listed as having a particle size of 0.004mm - 0.063mm. "Praf" is given as the name for unconsolidated deposits, but in the "categorii granulometrice" column it does say "silturi".
http://www.geo.edu.ro/~sedim/Petr-sed/Siliciclastite/Clasifi...

Now compare that with this definition of "silt" from the Britannia Concise Encyclopedia:

"silt

Sediment particles 0.00016 – 0.0024 in. **(0.004 – 0.06 mm)** in diameter, regardless of mineral type... **An unconsolidated aggregate of silt particles is also called silt**, whereas a consolidated aggregate is called siltstone. Silt deposits formed by wind are known as loess. **Sediments are seldom composed entirely of silt but rather are a mixture of clay, silt, and sand**."
http://www.answers.com/topic/silt?cat=technology

See also this American classification of particle sizes and terms, according to which "silt" would cover a very similar range of particle sizes:

0.02 - 0.05mm Coarse Silt

0.002 - 0.02mm Medium and Fine Silt
http://www.geog.ucl.ac.uk/~jhope/lab/partsize.htm

This UK classification (from DEFRA) uses "silt" for particles ranging in size from 0.002-0.06mm, which also corresponds very closely with the range covered by "praf" in my first link above:
http://www.defra.gov.uk/rds/publications/technical/tan_52.pd...

The term "dust" is apparently not used in either the US or UK classifications. See also http://en.wikipedia.org/wiki/Soil_texture_classification

Some examples of "silty clays" and "clayey silts":

"Geoarchaeological analysis
The geoarchaeological analysis showed sediments relating to fluvial estuarine and marine processes. One important alluvial sequence featuring silts and **clayey silts** with interbedded fine sands was targeted for environmental sampling... Sub-units identified within these major sedimentary units revealed repeated layers of fine sands and **silty clays** which could indicate repeated, possibly seasonal, flooding."
http://www.wessexarch.co.uk/projects/marine/alsf/seabed_preh...

"5.4.2. Terrace 2 (Hemington Terrace)

This terrace consists of **clayey silts and silty clays**.

0->70cm Description: clayey SILT and silty CLAY (see 5.3.2 and 5.3.3)
http://www.tvg.bham.ac.uk/alluvium_sourcing.pdf


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Note added at 7 hrs (2007-09-20 22:28:04 GMT)
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It certainly seems that "praf" means the same as "silt" in this document, since the terms are used interchangeably:

"...acestea fiind reprezentate atat prin loess - **silt (praf) argilos** - cat si prin **argila siltica (prafoasa)** interpretata ca reprezentand paleosoluri...

Conţinutul foarte ridicat de **silt (praf)** în argilele roşii (depăsind uneori 40%) explică prezenţa impregnării cu produse petroliere în acest orizont litologic."
http://66.102.9.104/search?q=cache:O-hMYxTFt2EJ:frf.cncsis.r...